From 80af79779d01fe949af84fdb43f6b82085cd4a9c Mon Sep 17 00:00:00 2001
From: rr- <rr-@sakuya.pl>
Date: Sun, 14 Aug 2016 20:06:49 +0200
Subject: [PATCH] server/snapshots: rewrite

---
 API.md                                        | 249 ++++++++-----
 server/migrate-v1                             |  45 ---
 server/szurubooru/api/post_api.py             |  18 +-
 server/szurubooru/api/snapshot_api.py         |   2 +-
 server/szurubooru/api/tag_api.py              |  10 +-
 server/szurubooru/api/tag_category_api.py     |   8 +-
 server/szurubooru/db/__init__.py              |  14 +-
 server/szurubooru/db/session.py               |  15 +-
 server/szurubooru/db/snapshot.py              |   9 +-
 server/szurubooru/db/util.py                  |  10 +-
 server/szurubooru/func/diff.py                |  57 +++
 server/szurubooru/func/posts.py               |   3 +-
 server/szurubooru/func/snapshots.py           | 175 ++++-----
 server/szurubooru/func/tag_categories.py      |   3 +-
 server/szurubooru/func/tags.py                |   3 +-
 .../4a020f1d271a_rename_snapshot_columns.py   |  54 +++
 .../search/configs/snapshot_search_config.py  |   2 +-
 .../tests/api/test_post_creating.py           |  11 +-
 .../tests/api/test_post_deleting.py           |  15 +-
 .../tests/api/test_post_featuring.py          |  15 +-
 .../tests/api/test_post_updating.py           |  13 +-
 .../tests/api/test_snapshot_retrieving.py     |   4 +-
 .../tests/api/test_tag_category_creating.py   |  23 +-
 .../tests/api/test_tag_category_deleting.py   |  15 +-
 .../tests/api/test_tag_category_retrieving.py |   1 -
 .../tests/api/test_tag_category_updating.py   |  14 +-
 .../szurubooru/tests/api/test_tag_creating.py |  10 +-
 .../szurubooru/tests/api/test_tag_deleting.py |  14 +-
 .../szurubooru/tests/api/test_tag_merging.py  |   8 +-
 .../szurubooru/tests/api/test_tag_updating.py |   5 +-
 server/szurubooru/tests/db/test_user.py       |   4 +-
 server/szurubooru/tests/func/test_diff.py     | 275 ++++++++++++++
 server/szurubooru/tests/func/test_posts.py    |   8 +-
 .../szurubooru/tests/func/test_snapshots.py   | 348 +++++++-----------
 .../tests/func/test_tag_categories.py         |  35 +-
 server/szurubooru/tests/func/test_tags.py     |  65 ++--
 36 files changed, 931 insertions(+), 629 deletions(-)
 create mode 100644 server/szurubooru/func/diff.py
 create mode 100644 server/szurubooru/migrations/versions/4a020f1d271a_rename_snapshot_columns.py
 create mode 100644 server/szurubooru/tests/func/test_diff.py

diff --git a/API.md b/API.md
index 44654be8..3c6bb2f5 100644
--- a/API.md
+++ b/API.md
@@ -943,10 +943,9 @@ data.
 - **Description**
 
     Retrieves the post that is currently featured on the main page in web
-    client. If no post is featured, `<post>` is null and `snapshots` array is
-    empty. Note that this method exists mostly for compatibility with setting
-    featured post - most of times, you'd want to use query global info which
-    contains more information.
+    client. If no post is featured, `<post>` is null. Note that this method
+    exists mostly for compatibility with setting featured post - most of times,
+    you'd want to use query global info which contains more information.
 
 ## Featuring post
 - **Request**
@@ -1441,7 +1440,7 @@ data.
     | `id`              | involving given resource id                   |
     | `date`            | created at given date                         |
     | `time`            | alias of `date`                               |
-    | `operation`       | `changed`, `created` or `deleted`             |
+    | `operation`       | `modified`, `created`, `deleted` or `merged`  |
     | `user`            | name of the user that created given snapshot  |
 
     **Sort style tokens**
@@ -1573,12 +1572,7 @@ experience.
     "name":    <name>,
     "color":   <color>,
     "usages":  <usages>
-    "default": <is-default>,
-    "snapshots": [
-        <snapshot>,
-        <snapshot>,
-        <snapshot>
-    ]
+    "default": <is-default>
 }
 ```
 
@@ -1589,8 +1583,6 @@ experience.
 - `<color>`: the category color.
 - `<usages>`: how many tags is the given category used with.
 - `<is-default>`: whether the tag category is the default one.
-- `<snapshot>`: a [snapshot resource](#snapshot) that contains the tag
-  category's earlier versions.
 
 ## Tag
 **Description**
@@ -1609,12 +1601,7 @@ A single tag. Tags are used to let users search for posts.
     "creationTime": <creation-time>,
     "lastEditTime": <last-edit-time>,
     "usages":       <usage-count>,
-    "description":  <description>,
-    "snapshots": [
-        <snapshot>,
-        <snapshot>,
-        <snapshot>
-    ]
+    "description":  <description>
 }
 ```
 
@@ -1633,8 +1620,6 @@ A single tag. Tags are used to let users search for posts.
 - `<usage-count>`: the number of posts the tag was used in.
 - `<description>`: the tag description (instructions how to use, history etc.)
   The client should render is as Markdown.
-- `<snapshot>`: a [snapshot resource](#snapshot) that contains the tag's
-  earlier versions.
 
 ## Post
 **Description**
@@ -1675,11 +1660,6 @@ One file together with its metadata posted to the site.
     "favoritedBy":        <favorited-by>,
     "hasCustomThumbnail": <has-custom-thumbnail>,
     "mimeType":           <mime-type>
-    "snapshots": [
-        <snapshot>,
-        <snapshot>,
-        <snapshot>
-    ],
     "comments": {
         <comment>,
         <comment>,
@@ -1745,8 +1725,6 @@ One file together with its metadata posted to the site.
 - `<has-custom-thumbnail>`: whether the post uses custom thumbnail.
 - `<mime-type>`: subsidiary to `<type>`, used to tell exact content format;
   useful for `<video>` tags for instance.
-- `<snapshot>`: a [snapshot resource](#snapshot) that contains the post's
-  earlier versions.
 - `<comment>`: a [comment resource](#comment) for given post.
 
 ## Micro post
@@ -1817,13 +1795,12 @@ A snapshot is a version of a database resource.
 
 ```json5
 {
-    "operation":    <operation>,
-    "type":         <resource-type>
-    "id":           <resource-id>,
-    "user":         <user-name>,
-    "data":         <data>,
-    "earlier-data": <earlier-data>,
-    "time":         <time>
+    "operation": <operation>,
+    "type":      <resource-type>,
+    "id":        <resource-id>,
+    "user":      <issuer>,
+    "data":      <data>,
+    "time":      <time>
 }
 ```
 
@@ -1836,6 +1813,7 @@ A snapshot is a version of a database resource.
     - `"created"` - the resource has been created
     - `"modified"` - the resource has been modified
     - `"deleted"` - the resource has been deleted
+    - `"merged"` - the resource has been merged to another resource
 
 - `<resource-type>` and `<resource-id>`: the resource that was changed.
 
@@ -1847,61 +1825,162 @@ A snapshot is a version of a database resource.
     | `"tag_category"`  | tag category name at given time |
     | `"post"`          | post ID                         |
 
-- `<user-name>`: name of the user who has made the change.
+- `<issuer>`: a [micro user resource](#micro-user) representing the user who
+    has made the change.
 
-- `<data>`: the snapshot data.
-
-    The value can be either of structures below:
-
-    - Tag category snapshot data (`<resource-type> = "tag"`)
-
-        *Example*
-
-        ```json5
-        {
-            "name":  "character",
-            "color": "#FF0000",
-            "default": false
-        }
-        ```
-
-    - Tag snapshot data (`<resource-type> = "tag"`)
-
-        *Example*
-
-        ```json5
-        {
-            "names":        ["tag1", "tag2", "tag3"],
-            "category":     "plain",
-            "implications": ["imp1", "imp2", "imp3"],
-            "suggestions":  ["sug1", "sug2", "sug3"]
-        }
-        ```
-
-    - Post snapshot data (`<resource-type> = "post"`)
-
-        *Example*
-
-        ```json5
-        {
-            "source": "http://example.com/",
-            "safety": "safe",
-            "checksum": "deadbeef",
-            "tags": ["tag1", "tag2"],
-            "relations": [1, 2],
-            "notes": [<note1>, <note2>, <note3>],
-            "flags": ["loop"],
-            "featured": false
-        }
-        ```
-
-- `<earlier-data>`: `<data>` field from the last snapshot of the same resource.
-  This allows the client to create visual diffs for any given snapshot without
-  the need to know any other snapshots for a given resource.
+- `<data>`: the snapshot data, of which content depends on the `<operation>`.
+   More explained later.
 
 - `<time>`: when the snapshot was created (i.e. when the resource was changed),
   formatted as per RFC 3339.
 
+**`<data>` field for creation snapshots**
+
+The value can be either of structures below, depending on
+`<resource-type>`:
+
+- Tag category snapshot data (`<resource-type> = "tag_category"`)
+
+    *Example*
+
+    ```json5
+    {
+        "name":  "character",
+        "color": "#FF0000",
+        "default": false
+    }
+    ```
+
+- Tag snapshot data (`<resource-type> = "tag"`)
+
+    *Example*
+
+    ```json5
+    {
+        "names":        ["tag1", "tag2", "tag3"],
+        "category":     "plain",
+        "implications": ["imp1", "imp2", "imp3"],
+        "suggestions":  ["sug1", "sug2", "sug3"]
+    }
+    ```
+
+- Post snapshot data (`<resource-type> = "post"`)
+
+    *Example*
+
+    ```json5
+    {
+        "source": "http://example.com/",
+        "safety": "safe",
+        "checksum": "deadbeef",
+        "tags": ["tag1", "tag2"],
+        "relations": [1, 2],
+        "notes": [<note1>, <note2>, <note3>],
+        "flags": ["loop"],
+        "featured": false
+    }
+    ```
+
+    `<note>`s are serialized the same way as [note resources](#note).
+
+**`<data>` field for modification snapshots**
+
+The value is a property-wise recursive diff between previous version of the
+resource and its current version. Its structure is a `<dictionary-diff>` of
+dictionaries as created by creation snapshots, which is described below.
+
+`<primitive>`: any primitive (number or a string)
+
+`<anything>`: any dictionary, list or primitive
+
+`<dictionary-diff>`:
+
+```json5
+{
+    "type": "object change",
+    "value":
+    {
+        "property-of-any-type-1":
+        {
+            "type": "deleted property",
+            "value": <anything>
+        },
+        "property-of-any-type-2":
+        {
+            "type": "added property",
+            "value": <anything>
+        },
+        "primitive-property":
+        {
+            "type": "primitive change":
+            "old-value": "<primitive>",
+            "new-value": "<primitive>"
+        },
+        "list-property": <list-diff>,
+        "dictionary-property": <dictionary-diff>
+    }
+}
+```
+
+`<list-diff>`:
+
+```json5
+{
+    "type": "list change",
+    "removed": [<anything>, <anything>],
+    "added": [<anything>, <anything>]
+}
+```
+
+Example - a diff for a post that has changed source and has one note added.
+Note the similarities with the structure of post creation snapshots.
+
+```json5
+{
+    "type": "object change",
+    "value":
+    {
+        "source":
+        {
+            "type": "primitive change",
+            "old-value": None,
+            "new-value": "new source"
+        },
+        "notes":
+        {
+            "type": "list change",
+            "removed": [],
+            "added":
+            [
+                {"polygon": [[0, 0], [0, 1], [1, 1]], "text": "new note"}
+            ]
+        }
+    }
+}
+```
+
+Since the snapshot dictionaries structure is pretty immutable, you probably
+won't see `added property` or `deleted property` around. This observation holds
+true even if the way the snapshots are generated changes - szurubooru stores
+just the diffs rather than original snapshots, so it wouldn't be able to
+generate a diff against an old version.
+
+**`<data>` field for deletion snapshots**
+
+Same as creation snapshot. In emergencies, it can be used to reconstruct
+deleted entities. Please note that this does not constitute as means against
+vandalism (it's still possible to cause chaos by mass editing - this should be
+dealt with by configuring role privileges in the config) or replace database
+backups.
+
+**`<data>` field for merge snapshots**
+
+A tuple containing 2 elements:
+
+- resource type equivalent to `<resource-type>` of the target entity.
+- resource ID euivalen to `<resource-id>` of the target entity.
+
+
 ## Unpaged search result
 **Description**
 
diff --git a/server/migrate-v1 b/server/migrate-v1
index 6c93b76a..0fdf9e4f 100755
--- a/server/migrate-v1
+++ b/server/migrate-v1
@@ -355,50 +355,6 @@ def import_scores(v1_session, v2_session):
         v2_session.add(score)
     v2_session.commit()
 
-def import_snapshots(v1_session, v2_session):
-    logger.info('Importing snapshots...')
-    for row in exec_query(v1_session, 'SELECT * FROM snapshots ORDER BY time ASC'):
-        snapshot = db.Snapshot()
-        snapshot.creation_time = row['time']
-        snapshot.user_id = row['userId']
-        snapshot.operation = {
-            0: db.Snapshot.OPERATION_CREATED,
-            1: db.Snapshot.OPERATION_MODIFIED,
-            2: db.Snapshot.OPERATION_DELETED,
-        }[row['operation']]
-        snapshot.resource_type = {
-            0: 'post',
-            1: 'tag',
-        }[row['type']]
-        snapshot.resource_id = row['primaryKey']
-
-        data = json.loads(zlib.decompress(row['data'], -15).decode('utf-8'))
-        if snapshot.resource_type == 'post':
-            if 'contentChecksum' in data:
-                data['checksum'] = data['contentChecksum']
-                del data['contentChecksum']
-            if 'tags' in data and isinstance(data['tags'], dict):
-                data['tags'] = list(data['tags'].values())
-            if 'notes' in data:
-                notes = []
-                for note in data['notes']:
-                    notes.append({
-                        'polygon': translate_note_polygon(note),
-                        'text': note['text'],
-                    })
-                data['notes'] = notes
-            snapshot.resource_repr = row['primaryKey']
-        elif snapshot.resource_type == 'tag':
-            if 'banned' in data:
-                del data['banned']
-            if 'name' in data:
-                data['names'] = [data['name']]
-                del data['name']
-            snapshot.resource_repr = data['names'][0]
-        snapshot.data = data
-        v2_session.add(snapshot)
-    v2_session.commit()
-
 def main():
     args = parse_args()
 
@@ -426,7 +382,6 @@ def main():
     import_post_favorites(unused_post_ids, v1_session, v2_session)
     import_comments(unused_post_ids, v1_session, v2_session)
     import_scores(v1_session, v2_session)
-    import_snapshots(v1_session, v2_session)
 
 if __name__ == '__main__':
     main()
diff --git a/server/szurubooru/api/post_api.py b/server/szurubooru/api/post_api.py
index 53bbcd26..b6613221 100644
--- a/server/szurubooru/api/post_api.py
+++ b/server/szurubooru/api/post_api.py
@@ -1,5 +1,5 @@
 import datetime
-from szurubooru import search
+from szurubooru import search, db
 from szurubooru.rest import routes
 from szurubooru.func import (
     auth, tags, posts, snapshots, favorites, scores, util, versions)
@@ -44,6 +44,9 @@ def create_post(ctx, _params=None):
         content, tag_names, None if anonymous else ctx.user)
     if len(new_tags):
         auth.verify_privilege(ctx.user, 'tags:create')
+        db.session.flush()
+        for tag in new_tags:
+            snapshots.create(tag, ctx.user)
     posts.update_post_safety(post, safety)
     posts.update_post_source(post, source)
     posts.update_post_relations(post, relations)
@@ -52,7 +55,7 @@ def create_post(ctx, _params=None):
     if ctx.has_file('thumbnail'):
         posts.update_post_thumbnail(post, ctx.get_file('thumbnail'))
     ctx.session.add(post)
-    snapshots.save_entity_creation(post, ctx.user)
+    snapshots.create(post, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize_post(ctx, post)
@@ -78,6 +81,9 @@ def update_post(ctx, params):
         new_tags = posts.update_post_tags(post, ctx.get_param_as_list('tags'))
         if len(new_tags):
             auth.verify_privilege(ctx.user, 'tags:create')
+            db.session.flush()
+            for tag in new_tags:
+                snapshots.create(tag, ctx.user)
     if ctx.has_param('safety'):
         auth.verify_privilege(ctx.user, 'posts:edit:safety')
         posts.update_post_safety(post, ctx.get_param_as_string('safety'))
@@ -100,7 +106,7 @@ def update_post(ctx, params):
         posts.update_post_thumbnail(post, ctx.get_file('thumbnail'))
     post.last_edit_time = datetime.datetime.utcnow()
     ctx.session.flush()
-    snapshots.save_entity_modification(post, ctx.user)
+    snapshots.modify(post, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize_post(ctx, post)
@@ -111,7 +117,7 @@ def delete_post(ctx, params):
     auth.verify_privilege(ctx.user, 'posts:delete')
     post = posts.get_post_by_id(params['post_id'])
     versions.verify_version(post, ctx)
-    snapshots.save_entity_deletion(post, ctx.user)
+    snapshots.delete(post, ctx.user)
     posts.delete(post)
     ctx.session.commit()
     tags.export_to_json()
@@ -134,9 +140,7 @@ def set_featured_post(ctx, _params=None):
         raise posts.PostAlreadyFeaturedError(
             'Post %r is already featured.' % post_id)
     posts.feature_post(post, ctx.user)
-    if featured_post:
-        snapshots.save_entity_modification(featured_post, ctx.user)
-    snapshots.save_entity_modification(post, ctx.user)
+    snapshots.modify(post, ctx.user)
     ctx.session.commit()
     return _serialize_post(ctx, post)
 
diff --git a/server/szurubooru/api/snapshot_api.py b/server/szurubooru/api/snapshot_api.py
index 1a5f5896..009d8c97 100644
--- a/server/szurubooru/api/snapshot_api.py
+++ b/server/szurubooru/api/snapshot_api.py
@@ -10,4 +10,4 @@ _search_executor = search.Executor(search.configs.SnapshotSearchConfig())
 def get_snapshots(ctx, _params=None):
     auth.verify_privilege(ctx.user, 'snapshots:list')
     return _search_executor.execute_and_serialize(
-        ctx, snapshots.serialize_snapshot)
+        ctx, lambda snapshot: snapshots.serialize_snapshot(snapshot, ctx.user))
diff --git a/server/szurubooru/api/tag_api.py b/server/szurubooru/api/tag_api.py
index 4daf7e5a..81872de0 100644
--- a/server/szurubooru/api/tag_api.py
+++ b/server/szurubooru/api/tag_api.py
@@ -20,7 +20,7 @@ def _create_if_needed(tag_names, user):
         auth.verify_privilege(user, 'tags:create')
     db.session.flush()
     for tag in new_tags:
-        snapshots.save_entity_creation(tag, user)
+        snapshots.create(tag, user)
 
 
 @routes.get('/tags/?')
@@ -50,7 +50,7 @@ def create_tag(ctx, _params=None):
     tags.update_tag_description(tag, description)
     ctx.session.add(tag)
     ctx.session.flush()
-    snapshots.save_entity_creation(tag, ctx.user)
+    snapshots.create(tag, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize(ctx, tag)
@@ -91,7 +91,7 @@ def update_tag(ctx, params):
         tags.update_tag_implications(tag, implications)
     tag.last_edit_time = datetime.datetime.utcnow()
     ctx.session.flush()
-    snapshots.save_entity_modification(tag, ctx.user)
+    snapshots.modify(tag, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize(ctx, tag)
@@ -102,7 +102,7 @@ def delete_tag(ctx, params):
     tag = tags.get_tag_by_name(params['tag_name'])
     versions.verify_version(tag, ctx)
     auth.verify_privilege(ctx.user, 'tags:delete')
-    snapshots.save_entity_deletion(tag, ctx.user)
+    snapshots.delete(tag, ctx.user)
     tags.delete(tag)
     ctx.session.commit()
     tags.export_to_json()
@@ -120,7 +120,7 @@ def merge_tags(ctx, _params=None):
     versions.bump_version(target_tag)
     auth.verify_privilege(ctx.user, 'tags:merge')
     tags.merge_tags(source_tag, target_tag)
-    snapshots.save_entity_deletion(source_tag, ctx.user)
+    snapshots.merge(source_tag, target_tag, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize(ctx, target_tag)
diff --git a/server/szurubooru/api/tag_category_api.py b/server/szurubooru/api/tag_category_api.py
index 27efb1c9..882af10c 100644
--- a/server/szurubooru/api/tag_category_api.py
+++ b/server/szurubooru/api/tag_category_api.py
@@ -25,7 +25,7 @@ def create_tag_category(ctx, _params=None):
     category = tag_categories.create_category(name, color)
     ctx.session.add(category)
     ctx.session.flush()
-    snapshots.save_entity_creation(category, ctx.user)
+    snapshots.create(category, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize(ctx, category)
@@ -52,7 +52,7 @@ def update_tag_category(ctx, params):
         tag_categories.update_category_color(
             category, ctx.get_param_as_string('color'))
     ctx.session.flush()
-    snapshots.save_entity_modification(category, ctx.user)
+    snapshots.modify(category, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize(ctx, category)
@@ -64,7 +64,7 @@ def delete_tag_category(ctx, params):
     versions.verify_version(category, ctx)
     auth.verify_privilege(ctx.user, 'tag_categories:delete')
     tag_categories.delete_category(category)
-    snapshots.save_entity_deletion(category, ctx.user)
+    snapshots.delete(category, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return {}
@@ -75,7 +75,7 @@ def set_tag_category_as_default(ctx, params):
     auth.verify_privilege(ctx.user, 'tag_categories:set_default')
     category = tag_categories.get_category_by_name(params['category_name'])
     tag_categories.set_default_category(category)
-    snapshots.save_entity_modification(category, ctx.user)
+    snapshots.modify(category, ctx.user)
     ctx.session.commit()
     tags.export_to_json()
     return _serialize(ctx, category)
diff --git a/server/szurubooru/db/__init__.py b/server/szurubooru/db/__init__.py
index 1dbed94a..3eb18833 100644
--- a/server/szurubooru/db/__init__.py
+++ b/server/szurubooru/db/__init__.py
@@ -1,11 +1,7 @@
 from szurubooru.db.base import Base
 from szurubooru.db.user import User
 from szurubooru.db.tag_category import TagCategory
-from szurubooru.db.tag import (
-    Tag,
-    TagName,
-    TagSuggestion,
-    TagImplication)
+from szurubooru.db.tag import (Tag, TagName, TagSuggestion, TagImplication)
 from szurubooru.db.post import (
     Post,
     PostTag,
@@ -14,12 +10,8 @@ from szurubooru.db.post import (
     PostScore,
     PostNote,
     PostFeature)
-from szurubooru.db.comment import (
-    Comment,
-    CommentScore)
+from szurubooru.db.comment import (Comment, CommentScore)
 from szurubooru.db.snapshot import Snapshot
 from szurubooru.db.session import (
-    session,
-    reset_query_count,
-    get_query_count)
+    session, sessionmaker, reset_query_count, get_query_count)
 import szurubooru.db.util
diff --git a/server/szurubooru/db/session.py b/server/szurubooru/db/session.py
index eb8f621c..98ad150b 100644
--- a/server/szurubooru/db/session.py
+++ b/server/szurubooru/db/session.py
@@ -18,15 +18,12 @@ class QueryCounter(object):
         return QueryCounter._query_count
 
 
-def create_session():
-    _engine = sqlalchemy.create_engine(config.config['database'])
-    sqlalchemy.event.listen(
-        _engine, 'after_execute', lambda *args: QueryCounter.bump())
-    _session_maker = sqlalchemy.orm.sessionmaker(bind=_engine)
-    return sqlalchemy.orm.scoped_session(_session_maker)
-
-
 # pylint: disable=invalid-name
-session = create_session()
+_engine = sqlalchemy.create_engine(config.config['database'])
+sessionmaker = sqlalchemy.orm.sessionmaker(bind=_engine)
+session = sqlalchemy.orm.scoped_session(sessionmaker)
 reset_query_count = QueryCounter.reset
 get_query_count = QueryCounter.get
+
+sqlalchemy.event.listen(
+    _engine, 'after_execute', lambda *args: QueryCounter.bump())
diff --git a/server/szurubooru/db/snapshot.py b/server/szurubooru/db/snapshot.py
index 45f1077a..4b211f61 100644
--- a/server/szurubooru/db/snapshot.py
+++ b/server/szurubooru/db/snapshot.py
@@ -10,14 +10,17 @@ class Snapshot(Base):
     OPERATION_CREATED = 'created'
     OPERATION_MODIFIED = 'modified'
     OPERATION_DELETED = 'deleted'
+    OPERATION_MERGED = 'merged'
 
     snapshot_id = Column('id', Integer, primary_key=True)
     creation_time = Column('creation_time', DateTime, nullable=False)
+    operation = Column('operation', Unicode(16), nullable=False)
     resource_type = Column(
         'resource_type', Unicode(32), nullable=False, index=True)
-    resource_id = Column('resource_id', Integer, nullable=False, index=True)
-    resource_repr = Column('resource_repr', Unicode(64), nullable=False)
-    operation = Column('operation', Unicode(16), nullable=False)
+    resource_pkey = Column(
+        'resource_pkey', Integer, nullable=False, index=True)
+    resource_name = Column(
+        'resource_name', Unicode(64), nullable=False)
     user_id = Column(
         'user_id',
         Integer,
diff --git a/server/szurubooru/db/util.py b/server/szurubooru/db/util.py
index 88ba6425..d6edf188 100644
--- a/server/szurubooru/db/util.py
+++ b/server/szurubooru/db/util.py
@@ -16,13 +16,13 @@ def get_resource_info(entity):
     assert primary_key is not None
     assert len(primary_key) == 1
 
-    resource_repr = serializers[resource_type](entity)
-    assert resource_repr
+    resource_name = serializers[resource_type](entity)
+    assert resource_name
 
-    resource_id = primary_key[0]
-    assert resource_id
+    resource_pkey = primary_key[0]
+    assert resource_pkey
 
-    return (resource_type, resource_id, resource_repr)
+    return (resource_type, resource_pkey, resource_name)
 
 
 def get_aux_entity(session, get_table_info, entity, user):
diff --git a/server/szurubooru/func/diff.py b/server/szurubooru/func/diff.py
new file mode 100644
index 00000000..0950f0f0
--- /dev/null
+++ b/server/szurubooru/func/diff.py
@@ -0,0 +1,57 @@
+def get_list_diff(old, new):
+    value = {'type': 'list change', 'added': [], 'removed': []}
+    equal = True
+
+    for item in old:
+        if item not in new:
+            equal = False
+            value['removed'].append(item)
+
+    for item in new:
+        if item not in old:
+            equal = False
+            value['added'].append(item)
+
+    return None if equal else value
+
+
+def get_dict_diff(old, new):
+    value = {}
+    equal = True
+
+    for key in old.keys():
+        if key in new:
+            if old[key] != new[key]:
+                if isinstance(old[key], dict) and isinstance(new[key], dict):
+                    value_diff = get_dict_diff(old[key], new[key])
+                    if value_diff:
+                        equal = False
+                        value[key] = value_diff
+                elif isinstance(old[key], list) and isinstance(new[key], list):
+                    value_diff = get_list_diff(old[key], new[key])
+                    if value_diff:
+                        equal = False
+                        value[key] = value_diff
+                else:
+                    equal = False
+                    value[key] = {
+                        'type': 'primitive change',
+                        'old-value': old[key],
+                        'new-value': new[key],
+                    }
+        else:
+            equal = False
+            value[key] = {
+                'type': 'deleted property',
+                'value': old[key]
+            }
+
+    for key in new.keys():
+        if key not in old:
+            equal = False
+            value[key] = {
+                'type': 'added property',
+                'value': new[key],
+            }
+
+    return None if equal else {'type': 'object change', 'value': value}
diff --git a/server/szurubooru/func/posts.py b/server/szurubooru/func/posts.py
index 027e7c4c..e79f1499 100644
--- a/server/szurubooru/func/posts.py
+++ b/server/szurubooru/func/posts.py
@@ -2,7 +2,7 @@ import datetime
 import sqlalchemy
 from szurubooru import config, db, errors
 from szurubooru.func import (
-    users, snapshots, scores, comments, tags, util, mime, images, files)
+    users, scores, comments, tags, util, mime, images, files)
 
 
 EMPTY_PIXEL = \
@@ -165,7 +165,6 @@ def serialize_post(post, auth_user, options=None):
                 for comment in sorted(
                     post.comments,
                     key=lambda comment: comment.creation_time)],
-            'snapshots': lambda: snapshots.get_serialized_history(post),
         },
         options)
 
diff --git a/server/szurubooru/func/snapshots.py b/server/szurubooru/func/snapshots.py
index a7543166..4c236bb5 100644
--- a/server/szurubooru/func/snapshots.py
+++ b/server/szurubooru/func/snapshots.py
@@ -1,8 +1,19 @@
-import datetime
+from datetime import datetime
 from szurubooru import db
+from szurubooru.func import diff, users
+
+
+def get_tag_category_snapshot(category):
+    assert category
+    return {
+        'name': category.name,
+        'color': category.color,
+        'default': True if category.default else False,
+    }
 
 
 def get_tag_snapshot(tag):
+    assert tag
     return {
         'names': [tag_name.name for tag_name in tag.names],
         'category': tag.category.name,
@@ -12,132 +23,106 @@ def get_tag_snapshot(tag):
 
 
 def get_post_snapshot(post):
+    assert post
     return {
         'source': post.source,
         'safety': post.safety,
         'checksum': post.checksum,
-        'tags': sorted([tag.first_name for tag in post.tags]),
-        'relations': sorted([
-            rel.post_id for rel in post.relations]),
-        'notes': sorted([{
-            'polygon': note.polygon,
-            'text': note.text,
-        } for note in post.notes], key=lambda x: x['polygon']),
         'flags': post.flags,
         'featured': post.is_featured,
+        'tags': sorted([tag.first_name for tag in post.tags]),
+        'relations': sorted([rel.post_id for rel in post.relations]),
+        'notes': sorted([{
+            'polygon': [[point[0], point[1]] for point in note.polygon],
+            'text': note.text,
+        } for note in post.notes], key=lambda x: x['polygon']),
     }
 
 
-def get_tag_category_snapshot(category):
-    return {
-        'name': category.name,
-        'color': category.color,
-        'default': True if category.default else False,
-    }
+_snapshot_factories = {
+    # lambdas allow mocking target functions in the tests
+    # pylint: disable=unnecessary-lambda
+    'tag_category': lambda entity: get_tag_category_snapshot(entity),
+    'tag': lambda entity: get_tag_snapshot(entity),
+    'post': lambda entity: get_post_snapshot(entity),
+}
 
 
-def get_previous_snapshot(snapshot):
+def serialize_snapshot(snapshot, auth_user):
     assert snapshot
-    return db.session \
-        .query(db.Snapshot) \
-        .filter(db.Snapshot.resource_type == snapshot.resource_type) \
-        .filter(db.Snapshot.resource_id == snapshot.resource_id) \
-        .filter(db.Snapshot.creation_time < snapshot.creation_time) \
-        .order_by(db.Snapshot.creation_time.desc()) \
-        .limit(1) \
-        .first()
-
-
-def get_snapshots(entity):
-    assert entity
-    resource_type, resource_id, _ = db.util.get_resource_info(entity)
-    return db.session \
-        .query(db.Snapshot) \
-        .filter(db.Snapshot.resource_type == resource_type) \
-        .filter(db.Snapshot.resource_id == resource_id) \
-        .order_by(db.Snapshot.creation_time.desc()) \
-        .all()
-
-
-def serialize_snapshot(snapshot, earlier_snapshot=()):
-    assert snapshot
-    if earlier_snapshot is ():
-        earlier_snapshot = get_previous_snapshot(snapshot)
     return {
         'operation': snapshot.operation,
         'type': snapshot.resource_type,
-        'id': snapshot.resource_repr,
-        'user': snapshot.user.name if snapshot.user else None,
+        'id': snapshot.resource_name,
+        'user': users.serialize_micro_user(snapshot.user, auth_user),
         'data': snapshot.data,
-        'earlier-data': earlier_snapshot.data if earlier_snapshot else None,
         'time': snapshot.creation_time,
     }
 
 
-def get_serialized_history(entity):
-    if not entity:
-        return []
-    ret = []
-    earlier_snapshot = None
-    for snapshot in reversed(get_snapshots(entity)):
-        ret.insert(0, serialize_snapshot(snapshot, earlier_snapshot))
-        earlier_snapshot = snapshot
-    return ret
-
-
-def _save(operation, entity, auth_user):
-    assert operation
-    assert entity
-    serializers = {
-        'tag': get_tag_snapshot,
-        'tag_category': get_tag_category_snapshot,
-        'post': get_post_snapshot,
-    }
-
-    resource_type, resource_id, resource_repr = (
+def _create(operation, entity, auth_user):
+    resource_type, resource_pkey, resource_name = (
         db.util.get_resource_info(entity))
-    now = datetime.datetime.utcnow()
 
     snapshot = db.Snapshot()
-    snapshot.creation_time = now
+    snapshot.creation_time = datetime.utcnow()
     snapshot.operation = operation
     snapshot.resource_type = resource_type
-    snapshot.resource_id = resource_id
-    snapshot.resource_repr = resource_repr
-    snapshot.data = serializers[resource_type](entity)
+    snapshot.resource_pkey = resource_pkey
+    snapshot.resource_name = resource_name
     snapshot.user = auth_user
-
-    earlier_snapshots = get_snapshots(entity)
-
-    delta = datetime.timedelta(minutes=10)
-    snapshots_left = len(earlier_snapshots)
-    while earlier_snapshots:
-        last_snapshot = earlier_snapshots.pop(0)
-        is_fresh = now - last_snapshot.creation_time <= delta
-        if snapshot.data != last_snapshot.data:
-            if not is_fresh or last_snapshot.user != auth_user:
-                break
-        db.session.delete(last_snapshot)
-        if snapshot.operation != db.Snapshot.OPERATION_DELETED:
-            snapshot.operation = last_snapshot.operation
-        snapshots_left -= 1
-
-    if not snapshots_left and operation == db.Snapshot.OPERATION_DELETED:
-        pass
-    else:
-        db.session.add(snapshot)
+    return snapshot
 
 
-def save_entity_creation(entity, auth_user):
+def create(entity, auth_user):
     assert entity
-    _save(db.Snapshot.OPERATION_CREATED, entity, auth_user)
+    snapshot = _create(db.Snapshot.OPERATION_CREATED, entity, auth_user)
+    snapshot_factory = _snapshot_factories[snapshot.resource_type]
+    snapshot.data = snapshot_factory(entity)
+    db.session.add(snapshot)
 
 
-def save_entity_modification(entity, auth_user):
+# pylint: disable=protected-access
+def modify(entity, auth_user):
     assert entity
-    _save(db.Snapshot.OPERATION_MODIFIED, entity, auth_user)
+
+    model = next((model
+        for model in db.Base._decl_class_registry.values()
+        if hasattr(model, '__table__')
+            and model.__table__.fullname == entity.__table__.fullname),
+        None)
+    assert model
+
+    snapshot = _create(db.Snapshot.OPERATION_MODIFIED, entity, auth_user)
+    snapshot_factory = _snapshot_factories[snapshot.resource_type]
+
+    detached_session = db.sessionmaker()
+    detached_entity = detached_session.query(model).get(snapshot.resource_pkey)
+    assert detached_entity, 'Entity not found in DB, have you committed it?'
+    detached_snapshot = snapshot_factory(detached_entity)
+    detached_session.close()
+
+    active_snapshot = snapshot_factory(entity)
+
+    snapshot.data = diff.get_dict_diff(detached_snapshot, active_snapshot)
+    if not snapshot.data:
+        return
+    db.session.add(snapshot)
 
 
-def save_entity_deletion(entity, auth_user):
+def delete(entity, auth_user):
     assert entity
-    _save(db.Snapshot.OPERATION_DELETED, entity, auth_user)
+    snapshot = _create(db.Snapshot.OPERATION_DELETED, entity, auth_user)
+    snapshot_factory = _snapshot_factories[snapshot.resource_type]
+    snapshot.data = snapshot_factory(entity)
+    db.session.add(snapshot)
+
+
+def merge(source_entity, target_entity, auth_user):
+    assert source_entity
+    assert target_entity
+    snapshot = _create(db.Snapshot.OPERATION_MERGED, source_entity, auth_user)
+    resource_type, _resource_pkey, resource_name = (
+        db.util.get_resource_info(target_entity))
+    snapshot.data = [resource_type, resource_name]
+    db.session.add(snapshot)
diff --git a/server/szurubooru/func/tag_categories.py b/server/szurubooru/func/tag_categories.py
index f476f332..ac27c999 100644
--- a/server/szurubooru/func/tag_categories.py
+++ b/server/szurubooru/func/tag_categories.py
@@ -1,7 +1,7 @@
 import re
 import sqlalchemy
 from szurubooru import config, db, errors
-from szurubooru.func import util, snapshots, cache
+from szurubooru.func import util, cache
 
 
 class TagCategoryNotFoundError(errors.NotFoundError):
@@ -40,7 +40,6 @@ def serialize_category(category, options=None):
             'color': lambda: category.color,
             'usages': lambda: category.tag_count,
             'default': lambda: category.default,
-            'snapshots': lambda: snapshots.get_serialized_history(category),
         },
         options)
 
diff --git a/server/szurubooru/func/tags.py b/server/szurubooru/func/tags.py
index e92440cd..22ebde33 100644
--- a/server/szurubooru/func/tags.py
+++ b/server/szurubooru/func/tags.py
@@ -4,7 +4,7 @@ import os
 import re
 import sqlalchemy
 from szurubooru import config, db, errors
-from szurubooru.func import util, tag_categories, snapshots
+from szurubooru.func import util, tag_categories
 
 
 class TagNotFoundError(errors.NotFoundError):
@@ -86,7 +86,6 @@ def serialize_tag(tag, options=None):
             'implications': lambda: [
                 relation.names[0].name
                 for relation in sort_tags(tag.implications)],
-            'snapshots': lambda: snapshots.get_serialized_history(tag),
         },
         options)
 
diff --git a/server/szurubooru/migrations/versions/4a020f1d271a_rename_snapshot_columns.py b/server/szurubooru/migrations/versions/4a020f1d271a_rename_snapshot_columns.py
new file mode 100644
index 00000000..e1c98ca9
--- /dev/null
+++ b/server/szurubooru/migrations/versions/4a020f1d271a_rename_snapshot_columns.py
@@ -0,0 +1,54 @@
+'''
+Rename snapshot columns
+
+Revision ID: 4a020f1d271a
+Created at: 2016-08-16 09:25:38.350861
+'''
+
+import sqlalchemy as sa
+from alembic import op
+
+
+revision = '4a020f1d271a'
+down_revision = '840b460c5613'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        'snapshot',
+        sa.Column('resource_name', sa.Unicode(length=64), nullable=False))
+    op.add_column(
+        'snapshot',
+        sa.Column('resource_pkey', sa.Integer(), nullable=False))
+    op.create_index(
+        op.f('ix_snapshot_resource_pkey'),
+        'snapshot',
+        ['resource_pkey'],
+        unique=False)
+    op.drop_index('ix_snapshot_resource_id', table_name='snapshot')
+    op.drop_column('snapshot', 'resource_id')
+    op.drop_column('snapshot', 'resource_repr')
+
+
+def downgrade():
+    op.add_column(
+        'snapshot',
+        sa.Column(
+            'resource_repr',
+            sa.VARCHAR(length=64),
+            autoincrement=False,
+            nullable=False))
+    op.add_column(
+        'snapshot',
+        sa.Column(
+            'resource_id',
+            sa.INTEGER(),
+            autoincrement=False,
+            nullable=False))
+    op.create_index(
+        'ix_snapshot_resource_id', 'snapshot', ['resource_id'], unique=False)
+    op.drop_index(op.f('ix_snapshot_resource_pkey'), table_name='snapshot')
+    op.drop_column('snapshot', 'resource_pkey')
+    op.drop_column('snapshot', 'resource_name')
diff --git a/server/szurubooru/search/configs/snapshot_search_config.py b/server/szurubooru/search/configs/snapshot_search_config.py
index a1a90d3e..6b1d51a8 100644
--- a/server/szurubooru/search/configs/snapshot_search_config.py
+++ b/server/szurubooru/search/configs/snapshot_search_config.py
@@ -14,7 +14,7 @@ class SnapshotSearchConfig(BaseSearchConfig):
     def named_filters(self):
         return {
             'type': search_util.create_str_filter(db.Snapshot.resource_type),
-            'id': search_util.create_str_filter(db.Snapshot.resource_repr),
+            'id': search_util.create_str_filter(db.Snapshot.resource_name),
             'date': search_util.create_date_filter(db.Snapshot.creation_time),
             'time': search_util.create_date_filter(db.Snapshot.creation_time),
             'operation': search_util.create_str_filter(db.Snapshot.operation),
diff --git a/server/szurubooru/tests/api/test_post_creating.py b/server/szurubooru/tests/api/test_post_creating.py
index 1a1f045f..d88cb9ae 100644
--- a/server/szurubooru/tests/api/test_post_creating.py
+++ b/server/szurubooru/tests/api/test_post_creating.py
@@ -31,7 +31,7 @@ def test_creating_minimal_posts(
             patch('szurubooru.func.posts.update_post_thumbnail'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_creation'):
+            patch('szurubooru.func.snapshots.create'):
         posts.create_post.return_value = (post, [])
         posts.serialize_post.return_value = 'serialized post'
 
@@ -61,8 +61,8 @@ def test_creating_minimal_posts(
             post, 'post-thumbnail')
         posts.serialize_post.assert_called_once_with(
             post, auth_user, options=None)
+        snapshots.create.assert_called_once_with(post, auth_user)
         tags.export_to_json.assert_called_once_with()
-        snapshots.save_entity_creation.assert_called_once_with(post, auth_user)
 
 
 def test_creating_full_posts(context_factory, post_factory, user_factory):
@@ -79,7 +79,7 @@ def test_creating_full_posts(context_factory, post_factory, user_factory):
             patch('szurubooru.func.posts.update_post_flags'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_creation'):
+            patch('szurubooru.func.snapshots.create'):
         posts.create_post.return_value = (post, [])
         posts.serialize_post.return_value = 'serialized post'
 
@@ -110,8 +110,8 @@ def test_creating_full_posts(context_factory, post_factory, user_factory):
             post, ['flag1', 'flag2'])
         posts.serialize_post.assert_called_once_with(
             post, auth_user, options=None)
+        snapshots.create.assert_called_once_with(post, auth_user)
         tags.export_to_json.assert_called_once_with()
-        snapshots.save_entity_creation.assert_called_once_with(post, auth_user)
 
 
 def test_anonymous_uploads(
@@ -122,7 +122,6 @@ def test_anonymous_uploads(
     db.session.flush()
 
     with patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_creation'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.posts.create_post'), \
             patch('szurubooru.func.posts.update_post_source'):
@@ -154,7 +153,6 @@ def test_creating_from_url_saves_source(
 
     with patch('szurubooru.func.net.download'), \
             patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_creation'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.posts.create_post'), \
             patch('szurubooru.func.posts.update_post_source'):
@@ -186,7 +184,6 @@ def test_creating_from_url_with_source_specified(
 
     with patch('szurubooru.func.net.download'), \
             patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_creation'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.posts.create_post'), \
             patch('szurubooru.func.posts.update_post_source'):
diff --git a/server/szurubooru/tests/api/test_post_deleting.py b/server/szurubooru/tests/api/test_post_deleting.py
index 28175802..19096e47 100644
--- a/server/szurubooru/tests/api/test_post_deleting.py
+++ b/server/szurubooru/tests/api/test_post_deleting.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import posts, tags
+from szurubooru.func import posts, tags, snapshots
 
 
 @pytest.fixture(autouse=True)
@@ -10,16 +10,17 @@ def inject_config(config_injector):
 
 
 def test_deleting(user_factory, post_factory, context_factory):
-    db.session.add(post_factory(id=1))
-    db.session.commit()
-    with patch('szurubooru.func.tags.export_to_json'):
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
+    post = post_factory(id=1)
+    db.session.add(post)
+    with patch('szurubooru.func.tags.export_to_json'), \
+            patch('szurubooru.func.snapshots.delete'):
         result = api.post_api.delete_post(
-            context_factory(
-                params={'version': 1},
-                user=user_factory(rank=db.User.RANK_REGULAR)),
+            context_factory(params={'version': 1}, user=auth_user),
             {'post_id': 1})
         assert result == {}
         assert db.session.query(db.Post).count() == 0
+        snapshots.delete.assert_called_once_with(post, auth_user)
         tags.export_to_json.assert_called_once_with()
 
 
diff --git a/server/szurubooru/tests/api/test_post_featuring.py b/server/szurubooru/tests/api/test_post_featuring.py
index 9f3bd0d3..c2822f8c 100644
--- a/server/szurubooru/tests/api/test_post_featuring.py
+++ b/server/szurubooru/tests/api/test_post_featuring.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import posts
+from szurubooru.func import posts, snapshots
 
 
 @pytest.fixture(autouse=True)
@@ -15,15 +15,15 @@ def inject_config(config_injector):
 
 
 def test_featuring(user_factory, post_factory, context_factory):
-    db.session.add(post_factory(id=1))
-    db.session.commit()
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
+    post = post_factory(id=1)
+    db.session.add(post)
     assert not posts.get_post_by_id(1).is_featured
-    with patch('szurubooru.func.posts.serialize_post'):
+    with patch('szurubooru.func.posts.serialize_post'), \
+            patch('szurubooru.func.snapshots.modify'):
         posts.serialize_post.return_value = 'serialized post'
         result = api.post_api.set_featured_post(
-            context_factory(
-                params={'id': 1},
-                user=user_factory(rank=db.User.RANK_REGULAR)))
+            context_factory(params={'id': 1}, user=auth_user))
         assert result == 'serialized post'
         assert posts.try_get_featured_post() is not None
         assert posts.try_get_featured_post().post_id == 1
@@ -32,6 +32,7 @@ def test_featuring(user_factory, post_factory, context_factory):
             context_factory(
                 user=user_factory(rank=db.User.RANK_REGULAR)))
         assert result == 'serialized post'
+        snapshots.modify.assert_called_once_with(post, auth_user)
 
 
 def test_trying_to_omit_required_parameter(user_factory, context_factory):
diff --git a/server/szurubooru/tests/api/test_post_updating.py b/server/szurubooru/tests/api/test_post_updating.py
index b577c7ee..790e835e 100644
--- a/server/szurubooru/tests/api/test_post_updating.py
+++ b/server/szurubooru/tests/api/test_post_updating.py
@@ -40,7 +40,7 @@ def test_post_updating(
             patch('szurubooru.func.posts.update_post_flags'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_modification'), \
+            patch('szurubooru.func.snapshots.modify'), \
             fake_datetime('1997-01-01'):
         posts.serialize_post.return_value = 'serialized post'
 
@@ -77,9 +77,8 @@ def test_post_updating(
             post, ['flag1', 'flag2'])
         posts.serialize_post.assert_called_once_with(
             post, auth_user, options=None)
+        snapshots.modify.assert_called_once_with(post, auth_user)
         tags.export_to_json.assert_called_once_with()
-        snapshots.save_entity_modification.assert_called_once_with(
-            post, auth_user)
         assert post.last_edit_time == datetime(1997, 1, 1)
 
 
@@ -90,10 +89,10 @@ def test_uploading_from_url_saves_source(
     db.session.flush()
     with patch('szurubooru.func.net.download'), \
             patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_modification'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.posts.update_post_content'), \
-            patch('szurubooru.func.posts.update_post_source'):
+            patch('szurubooru.func.posts.update_post_source'), \
+            patch('szurubooru.func.snapshots.modify'):
         net.download.return_value = b'content'
         api.post_api.update_post(
             context_factory(
@@ -112,10 +111,10 @@ def test_uploading_from_url_with_source_specified(
     db.session.flush()
     with patch('szurubooru.func.net.download'), \
             patch('szurubooru.func.tags.export_to_json'), \
-            patch('szurubooru.func.snapshots.save_entity_modification'), \
             patch('szurubooru.func.posts.serialize_post'), \
             patch('szurubooru.func.posts.update_post_content'), \
-            patch('szurubooru.func.posts.update_post_source'):
+            patch('szurubooru.func.posts.update_post_source'), \
+            patch('szurubooru.func.snapshots.modify'):
         net.download.return_value = b'content'
         api.post_api.update_post(
             context_factory(
diff --git a/server/szurubooru/tests/api/test_snapshot_retrieving.py b/server/szurubooru/tests/api/test_snapshot_retrieving.py
index a04326b8..7e48d10b 100644
--- a/server/szurubooru/tests/api/test_snapshot_retrieving.py
+++ b/server/szurubooru/tests/api/test_snapshot_retrieving.py
@@ -7,8 +7,8 @@ def snapshot_factory():
     snapshot = db.Snapshot()
     snapshot.creation_time = datetime(1999, 1, 1)
     snapshot.resource_type = 'dummy'
-    snapshot.resource_id = 1
-    snapshot.resource_repr = 'dummy'
+    snapshot.resource_pkey = 1
+    snapshot.resource_name = 'dummy'
     snapshot.operation = 'added'
     snapshot.data = '{}'
     return snapshot
diff --git a/server/szurubooru/tests/api/test_tag_category_creating.py b/server/szurubooru/tests/api/test_tag_category_creating.py
index 8bf0792b..96afc390 100644
--- a/server/szurubooru/tests/api/test_tag_category_creating.py
+++ b/server/szurubooru/tests/api/test_tag_category_creating.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import tag_categories, tags
+from szurubooru.func import tag_categories, tags, snapshots
 
 
 def _update_category_name(category, name):
@@ -15,21 +15,26 @@ def inject_config(config_injector):
     })
 
 
-def test_creating_category(user_factory, context_factory):
-    with patch('szurubooru.func.tag_categories.serialize_category'), \
+def test_creating_category(
+        tag_category_factory, user_factory, context_factory):
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
+    category = tag_category_factory(name='meta')
+    db.session.add(category)
+
+    with patch('szurubooru.func.tag_categories.create_category'), \
+            patch('szurubooru.func.tag_categories.serialize_category'), \
             patch('szurubooru.func.tag_categories.update_category_name'), \
+            patch('szurubooru.func.snapshots.create'), \
             patch('szurubooru.func.tags.export_to_json'):
+        tag_categories.create_category.return_value = category
         tag_categories.update_category_name.side_effect = _update_category_name
         tag_categories.serialize_category.return_value = 'serialized category'
         result = api.tag_category_api.create_tag_category(
             context_factory(
-                params={'name': 'meta', 'color': 'black'},
-                user=user_factory(rank=db.User.RANK_REGULAR)))
+                params={'name': 'meta', 'color': 'black'}, user=auth_user))
         assert result == 'serialized category'
-        category = db.session.query(db.TagCategory).one()
-        assert category.name == 'meta'
-        assert category.color == 'black'
-        assert category.tag_count == 0
+        tag_categories.create_category.assert_called_once_with('meta', 'black')
+        snapshots.create.assert_called_once_with(category, auth_user)
         tags.export_to_json.assert_called_once_with()
 
 
diff --git a/server/szurubooru/tests/api/test_tag_category_deleting.py b/server/szurubooru/tests/api/test_tag_category_deleting.py
index bb590efb..0a8d59bf 100644
--- a/server/szurubooru/tests/api/test_tag_category_deleting.py
+++ b/server/szurubooru/tests/api/test_tag_category_deleting.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import tag_categories, tags
+from szurubooru.func import tag_categories, tags, snapshots
 
 
 @pytest.fixture(autouse=True)
@@ -12,18 +12,19 @@ def inject_config(config_injector):
 
 
 def test_deleting(user_factory, tag_category_factory, context_factory):
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
+    category = tag_category_factory(name='category')
     db.session.add(tag_category_factory(name='root'))
-    db.session.add(tag_category_factory(name='category'))
-    db.session.commit()
-    with patch('szurubooru.func.tags.export_to_json'):
+    db.session.add(category)
+    with patch('szurubooru.func.snapshots.delete'), \
+            patch('szurubooru.func.tags.export_to_json'):
         result = api.tag_category_api.delete_tag_category(
-            context_factory(
-                params={'version': 1},
-                user=user_factory(rank=db.User.RANK_REGULAR)),
+            context_factory(params={'version': 1}, user=auth_user),
             {'category_name': 'category'})
         assert result == {}
         assert db.session.query(db.TagCategory).count() == 1
         assert db.session.query(db.TagCategory).one().name == 'root'
+        snapshots.delete.assert_called_once_with(category, auth_user)
         tags.export_to_json.assert_called_once_with()
 
 
diff --git a/server/szurubooru/tests/api/test_tag_category_retrieving.py b/server/szurubooru/tests/api/test_tag_category_retrieving.py
index e3715308..7a834f04 100644
--- a/server/szurubooru/tests/api/test_tag_category_retrieving.py
+++ b/server/szurubooru/tests/api/test_tag_category_retrieving.py
@@ -35,7 +35,6 @@ def test_retrieving_single(
         'color': 'dummy',
         'usages': 0,
         'default': False,
-        'snapshots': [],
         'version': 1,
     }
 
diff --git a/server/szurubooru/tests/api/test_tag_category_updating.py b/server/szurubooru/tests/api/test_tag_category_updating.py
index d5a863a4..823d59ab 100644
--- a/server/szurubooru/tests/api/test_tag_category_updating.py
+++ b/server/szurubooru/tests/api/test_tag_category_updating.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import tag_categories, tags
+from szurubooru.func import tag_categories, tags, snapshots
 
 
 def _update_category_name(category, name):
@@ -20,29 +20,27 @@ def inject_config(config_injector):
 
 
 def test_simple_updating(user_factory, tag_category_factory, context_factory):
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
     category = tag_category_factory(name='name', color='black')
     db.session.add(category)
-    db.session.commit()
     with patch('szurubooru.func.tag_categories.serialize_category'), \
             patch('szurubooru.func.tag_categories.update_category_name'), \
             patch('szurubooru.func.tag_categories.update_category_color'), \
+            patch('szurubooru.func.snapshots.modify'), \
             patch('szurubooru.func.tags.export_to_json'):
         tag_categories.update_category_name.side_effect = _update_category_name
         tag_categories.serialize_category.return_value = 'serialized category'
         result = api.tag_category_api.update_tag_category(
             context_factory(
-                params={
-                    'name': 'changed',
-                    'color': 'white',
-                    'version': 1,
-                },
-                user=user_factory(rank=db.User.RANK_REGULAR)),
+                params={'name': 'changed', 'color': 'white', 'version': 1},
+                user=auth_user),
             {'category_name': 'name'})
         assert result == 'serialized category'
         tag_categories.update_category_name.assert_called_once_with(
             category, 'changed')
         tag_categories.update_category_color.assert_called_once_with(
             category, 'white')
+        snapshots.modify.assert_called_once_with(category, auth_user)
         tags.export_to_json.assert_called_once_with()
 
 
diff --git a/server/szurubooru/tests/api/test_tag_creating.py b/server/szurubooru/tests/api/test_tag_creating.py
index 2c551774..dc056280 100644
--- a/server/szurubooru/tests/api/test_tag_creating.py
+++ b/server/szurubooru/tests/api/test_tag_creating.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import tags
+from szurubooru.func import tags, snapshots
 
 
 @pytest.fixture(autouse=True)
@@ -10,12 +10,15 @@ def inject_config(config_injector):
 
 
 def test_creating_simple_tags(tag_factory, user_factory, context_factory):
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
+    tag = tag_factory()
     with patch('szurubooru.func.tags.create_tag'), \
             patch('szurubooru.func.tags.get_or_create_tags_by_names'), \
             patch('szurubooru.func.tags.serialize_tag'), \
+            patch('szurubooru.func.snapshots.create'), \
             patch('szurubooru.func.tags.export_to_json'):
         tags.get_or_create_tags_by_names.return_value = ([], [])
-        tags.create_tag.return_value = tag_factory()
+        tags.create_tag.return_value = tag
         tags.serialize_tag.return_value = 'serialized tag'
         result = api.tag_api.create_tag(
             context_factory(
@@ -26,10 +29,11 @@ def test_creating_simple_tags(tag_factory, user_factory, context_factory):
                     'suggestions': ['sug1', 'sug2'],
                     'implications': ['imp1', 'imp2'],
                 },
-                user=user_factory(rank=db.User.RANK_REGULAR)))
+                user=auth_user))
         assert result == 'serialized tag'
         tags.create_tag.assert_called_once_with(
             ['tag1', 'tag2'], 'meta', ['sug1', 'sug2'], ['imp1', 'imp2'])
+        snapshots.create.assert_called_once_with(tag, auth_user)
         tags.export_to_json.assert_called_once_with()
 
 
diff --git a/server/szurubooru/tests/api/test_tag_deleting.py b/server/szurubooru/tests/api/test_tag_deleting.py
index b5237d1c..a657b02e 100644
--- a/server/szurubooru/tests/api/test_tag_deleting.py
+++ b/server/szurubooru/tests/api/test_tag_deleting.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import tags
+from szurubooru.func import tags, snapshots
 
 
 @pytest.fixture(autouse=True)
@@ -10,16 +10,18 @@ def inject_config(config_injector):
 
 
 def test_deleting(user_factory, tag_factory, context_factory):
-    db.session.add(tag_factory(names=['tag']))
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
+    tag = tag_factory(names=['tag'])
+    db.session.add(tag)
     db.session.commit()
-    with patch('szurubooru.func.tags.export_to_json'):
+    with patch('szurubooru.func.tags.export_to_json'), \
+            patch('szurubooru.func.snapshots.delete'):
         result = api.tag_api.delete_tag(
-            context_factory(
-                params={'version': 1},
-                user=user_factory(rank=db.User.RANK_REGULAR)),
+            context_factory(params={'version': 1}, user=auth_user),
             {'tag_name': 'tag'})
         assert result == {}
         assert db.session.query(db.Tag).count() == 0
+        snapshots.delete.assert_called_once_with(tag, auth_user)
         tags.export_to_json.assert_called_once_with()
 
 
diff --git a/server/szurubooru/tests/api/test_tag_merging.py b/server/szurubooru/tests/api/test_tag_merging.py
index 9786abcb..a448c9c4 100644
--- a/server/szurubooru/tests/api/test_tag_merging.py
+++ b/server/szurubooru/tests/api/test_tag_merging.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import tags
+from szurubooru.func import tags, snapshots
 
 
 @pytest.fixture(autouse=True)
@@ -10,6 +10,7 @@ def inject_config(config_injector):
 
 
 def test_merging(user_factory, tag_factory, context_factory, post_factory):
+    auth_user = user_factory(rank=db.User.RANK_REGULAR)
     source_tag = tag_factory(names=['source'])
     target_tag = tag_factory(names=['target'])
     db.session.add_all([source_tag, target_tag])
@@ -24,6 +25,7 @@ def test_merging(user_factory, tag_factory, context_factory, post_factory):
     assert target_tag.post_count == 0
     with patch('szurubooru.func.tags.serialize_tag'), \
             patch('szurubooru.func.tags.merge_tags'), \
+            patch('szurubooru.func.snapshots.merge'), \
             patch('szurubooru.func.tags.export_to_json'):
         api.tag_api.merge_tags(
             context_factory(
@@ -33,8 +35,10 @@ def test_merging(user_factory, tag_factory, context_factory, post_factory):
                     'remove': 'source',
                     'mergeTo': 'target',
                 },
-                user=user_factory(rank=db.User.RANK_REGULAR)))
+                user=auth_user))
         tags.merge_tags.called_once_with(source_tag, target_tag)
+        snapshots.merge.assert_called_once_with(
+            source_tag, target_tag, auth_user)
         tags.export_to_json.assert_called_once_with()
 
 
diff --git a/server/szurubooru/tests/api/test_tag_updating.py b/server/szurubooru/tests/api/test_tag_updating.py
index 81f195b9..3fe69bd8 100644
--- a/server/szurubooru/tests/api/test_tag_updating.py
+++ b/server/szurubooru/tests/api/test_tag_updating.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import api, db, errors
-from szurubooru.func import tags
+from szurubooru.func import tags, snapshots
 
 
 @pytest.fixture(autouse=True)
@@ -31,6 +31,7 @@ def test_simple_updating(user_factory, tag_factory, context_factory):
             patch('szurubooru.func.tags.update_tag_suggestions'), \
             patch('szurubooru.func.tags.update_tag_implications'), \
             patch('szurubooru.func.tags.serialize_tag'), \
+            patch('szurubooru.func.snapshots.modify'), \
             patch('szurubooru.func.tags.export_to_json'):
         tags.get_or_create_tags_by_names.return_value = ([], [])
         tags.serialize_tag.return_value = 'serialized tag'
@@ -57,6 +58,8 @@ def test_simple_updating(user_factory, tag_factory, context_factory):
             tag, ['imp1', 'imp2'])
         tags.serialize_tag.assert_called_once_with(
             tag, options=None)
+        snapshots.modify.assert_called_once_with(tag, auth_user)
+        tags.export_to_json.assert_called_once_with()
 
 
 @pytest.mark.parametrize(
diff --git a/server/szurubooru/tests/db/test_user.py b/server/szurubooru/tests/db/test_user.py
index e63ad619..59933e36 100644
--- a/server/szurubooru/tests/db/test_user.py
+++ b/server/szurubooru/tests/db/test_user.py
@@ -145,8 +145,8 @@ def test_cascade_deletions(post_factory, user_factory, comment_factory):
     snapshot.user = user
     snapshot.creation_time = datetime(1997, 1, 1)
     snapshot.resource_type = '-'
-    snapshot.resource_id = 1
-    snapshot.resource_repr = '-'
+    snapshot.resource_pkey = 1
+    snapshot.resource_name = '-'
     snapshot.operation = '-'
 
     db.session.add_all([user, post, comment, snapshot])
diff --git a/server/szurubooru/tests/func/test_diff.py b/server/szurubooru/tests/func/test_diff.py
new file mode 100644
index 00000000..0134a3fd
--- /dev/null
+++ b/server/szurubooru/tests/func/test_diff.py
@@ -0,0 +1,275 @@
+import pytest
+from szurubooru.func import diff
+
+
+@pytest.mark.parametrize('old,new,expected', [
+    (
+        [], [], None,
+    ),
+
+    (
+        [],
+        ['added'],
+        {'type': 'list change', 'added': ['added'], 'removed': []},
+    ),
+
+    (
+        ['removed'],
+        [],
+        {'type': 'list change', 'added': [], 'removed': ['removed']},
+    ),
+
+    (
+        ['untouched'],
+        ['untouched'],
+        None,
+    ),
+
+    (
+        ['untouched'],
+        ['untouched', 'added'],
+        {'type': 'list change', 'added': ['added'], 'removed': []},
+    ),
+
+    (
+        ['untouched', 'removed'],
+        ['untouched'],
+        {'type': 'list change', 'added': [], 'removed': ['removed']},
+    ),
+])
+def test_get_list_diff(old, new, expected):
+    assert diff.get_list_diff(old, new) == expected
+
+
+@pytest.mark.parametrize('old,new,expected', [
+    (
+        {}, {}, None,
+    ),
+
+    (
+        {'removed key': 'removed value'},
+        {},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'removed key':
+                {
+                    'type': 'deleted property',
+                    'value': 'removed value',
+                },
+            },
+        },
+    ),
+
+    (
+        {},
+        {'added key': 'added value'},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'added key':
+                {
+                    'type': 'added property',
+                    'value': 'added value',
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': 'old value'},
+        {'key': 'new value'},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'key':
+                {
+                    'type': 'primitive change',
+                    'old-value': 'old value',
+                    'new-value': 'new value',
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': 'untouched'},
+        {'key': 'untouched'},
+        None,
+    ),
+
+    (
+        {'key': 'untouched', 'removed key': 'removed value'},
+        {'key': 'untouched'},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'removed key':
+                {
+                    'type': 'deleted property',
+                    'value': 'removed value',
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': 'untouched'},
+        {'key': 'untouched', 'added key': 'added value'},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'added key':
+                {
+                    'type': 'added property',
+                    'value': 'added value',
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': 'untouched', 'changed key': 'old value'},
+        {'key': 'untouched', 'changed key': 'new value'},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'changed key':
+                {
+                    'type': 'primitive change',
+                    'old-value': 'old value',
+                    'new-value': 'new value',
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': {'subkey': 'old value'}},
+        {'key': {'subkey': 'new value'}},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'key':
+                {
+                    'type': 'object change',
+                    'value':
+                    {
+                        'subkey':
+                        {
+                            'type': 'primitive change',
+                            'old-value': 'old value',
+                            'new-value': 'new value',
+                        },
+                    },
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': {}},
+        {'key': {'subkey': 'removed value'}},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'key':
+                {
+                    'type': 'object change',
+                    'value':
+                    {
+                        'subkey':
+                        {
+                            'type': 'added property',
+                            'value': 'removed value',
+                        },
+                    },
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': {'subkey': 'removed value'}},
+        {'key': {}},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'key':
+                {
+                    'type': 'object change',
+                    'value':
+                    {
+                        'subkey':
+                        {
+                            'type': 'deleted property',
+                            'value': 'removed value',
+                        },
+                    },
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': ['old value']},
+        {'key': ['new value']},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'key':
+                {
+                    'type': 'list change',
+                    'added': ['new value'],
+                    'removed': ['old value'],
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': []},
+        {'key': ['new value']},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'key':
+                {
+                    'type': 'list change',
+                    'added': ['new value'],
+                    'removed': [],
+                },
+            },
+        },
+    ),
+
+    (
+        {'key': ['removed value']},
+        {'key': []},
+        {
+            'type': 'object change',
+            'value':
+            {
+                'key':
+                {
+                    'type': 'list change',
+                    'added': [],
+                    'removed': ['removed value'],
+                },
+            },
+        },
+    ),
+])
+def test_get_dict_diff(old, new, expected):
+    assert diff.get_dict_diff(old, new) == expected
diff --git a/server/szurubooru/tests/func/test_posts.py b/server/szurubooru/tests/func/test_posts.py
index a2b91693..43581b69 100644
--- a/server/szurubooru/tests/func/test_posts.py
+++ b/server/szurubooru/tests/func/test_posts.py
@@ -3,8 +3,7 @@ from unittest.mock import patch
 from datetime import datetime
 import pytest
 from szurubooru import db
-from szurubooru.func import (
-    posts, users, comments, snapshots, tags, images, files, util)
+from szurubooru.func import (posts, users, comments, tags, images, files, util)
 
 
 @pytest.mark.parametrize('input_mime_type,expected_url', [
@@ -78,14 +77,12 @@ def test_serialize_post(
     config_injector({'data_url': 'http://example.com/'})
     with patch('szurubooru.func.comments.serialize_comment'), \
             patch('szurubooru.func.users.serialize_micro_user'), \
-            patch('szurubooru.func.posts.files.has'), \
-            patch('szurubooru.func.snapshots.get_serialized_history'):
+            patch('szurubooru.func.posts.files.has'):
         files.has.return_value = True
         users.serialize_micro_user.side_effect \
             = lambda user, auth_user: user.name
         comments.serialize_comment.side_effect \
             = lambda comment, auth_user: comment.user.name
-        snapshots.get_serialized_history.return_value = 'snapshot history'
 
         auth_user = user_factory(name='auth user')
         post = db.Post()
@@ -178,7 +175,6 @@ def test_serialize_post(
             'favoritedBy': ['fav1'],
             'hasCustomThumbnail': True,
             'mimeType': 'image/jpeg',
-            'snapshots': 'snapshot history',
             'comments': ['commenter1', 'commenter2'],
         }
 
diff --git a/server/szurubooru/tests/func/test_snapshots.py b/server/szurubooru/tests/func/test_snapshots.py
index 0ad17bad..2577d006 100644
--- a/server/szurubooru/tests/func/test_snapshots.py
+++ b/server/szurubooru/tests/func/test_snapshots.py
@@ -1,10 +1,52 @@
+from unittest.mock import patch
 from datetime import datetime
 import pytest
 from szurubooru import db
-from szurubooru.func import snapshots
+from szurubooru.func import snapshots, users
 
 
-def test_serializing_post(post_factory, user_factory, tag_factory):
+def test_get_tag_category_snapshot(tag_category_factory):
+    category = tag_category_factory(name='name', color='color')
+    assert snapshots.get_tag_category_snapshot(category) == {
+        'name': 'name',
+        'color': 'color',
+        'default': False,
+    }
+    category.default = True
+    assert snapshots.get_tag_category_snapshot(category) == {
+        'name': 'name',
+        'color': 'color',
+        'default': True,
+    }
+
+
+def test_get_tag_snapshot(tag_factory, tag_category_factory):
+    category = tag_category_factory(name='dummy')
+    tag = tag_factory(names=['main_name', 'alias'], category=category)
+    assert snapshots.get_tag_snapshot(tag) == {
+        'names': ['main_name', 'alias'],
+        'category': 'dummy',
+        'suggestions': [],
+        'implications': [],
+    }
+    tag = tag_factory(names=['main_name', 'alias'], category=category)
+    imp1 = tag_factory(names=['imp1_main_name', 'imp1_alias'])
+    imp2 = tag_factory(names=['imp2_main_name', 'imp2_alias'])
+    sug1 = tag_factory(names=['sug1_main_name', 'sug1_alias'])
+    sug2 = tag_factory(names=['sug2_main_name', 'sug2_alias'])
+    db.session.add_all([imp1, imp2, sug1, sug2])
+    tag.implications = [imp1, imp2]
+    tag.suggestions = [sug1, sug2]
+    db.session.flush()
+    assert snapshots.get_tag_snapshot(tag) == {
+        'names': ['main_name', 'alias'],
+        'category': 'dummy',
+        'implications': ['imp1_main_name', 'imp2_main_name'],
+        'suggestions': ['sug1_main_name', 'sug2_main_name'],
+    }
+
+
+def test_get_post_snapshot(post_factory, user_factory, tag_factory):
     user = user_factory(name='dummy-user')
     tag1 = tag_factory(names=['dummy-tag1'])
     tag2 = tag_factory(names=['dummy-tag2'])
@@ -50,12 +92,10 @@ def test_serializing_post(post_factory, user_factory, tag_factory):
         'checksum': 'deadbeef',
         'featured': True,
         'flags': [],
-        'notes': [
-            {
-                'polygon': [(1, 1), (200, 1), (200, 200), (1, 200)],
-                'text': 'some text',
-            }
-        ],
+        'notes': [{
+            'polygon': [[1, 1], [200, 1], [200, 200], [1, 200]],
+            'text': 'some text',
+        }],
         'relations': [2, 3],
         'safety': 'safe',
         'source': 'example.com',
@@ -63,243 +103,103 @@ def test_serializing_post(post_factory, user_factory, tag_factory):
     }
 
 
-def test_serializing_tag(tag_factory, tag_category_factory):
-    category = tag_category_factory(name='dummy')
-    tag = tag_factory(names=['main_name', 'alias'], category=category)
-    assert snapshots.get_tag_snapshot(tag) == {
-        'names': ['main_name', 'alias'],
-        'category': 'dummy',
-        'suggestions': [],
-        'implications': [],
-    }
-
-    tag = tag_factory(names=['main_name', 'alias'], category=category)
-    imp1 = tag_factory(names=['imp1_main_name', 'imp1_alias'])
-    imp2 = tag_factory(names=['imp2_main_name', 'imp2_alias'])
-    sug1 = tag_factory(names=['sug1_main_name', 'sug1_alias'])
-    sug2 = tag_factory(names=['sug2_main_name', 'sug2_alias'])
-    db.session.add_all([imp1, imp2, sug1, sug2])
-    tag.implications = [imp1, imp2]
-    tag.suggestions = [sug1, sug2]
-    db.session.flush()
-    assert snapshots.get_tag_snapshot(tag) == {
-        'names': ['main_name', 'alias'],
-        'category': 'dummy',
-        'implications': ['imp1_main_name', 'imp2_main_name'],
-        'suggestions': ['sug1_main_name', 'sug2_main_name'],
-    }
+def test_serialize_snapshot(user_factory):
+    auth_user = user_factory()
+    snapshot = db.Snapshot()
+    snapshot.operation = snapshot.OPERATION_CREATED
+    snapshot.resource_type = 'type'
+    snapshot.resource_name = 'id'
+    snapshot.user = user_factory(name='issuer')
+    snapshot.data = {'complex': list('object')}
+    snapshot.creation_time = datetime(1997, 1, 1)
+    with patch('szurubooru.func.users.serialize_micro_user'):
+        users.serialize_micro_user.return_value = 'mocked'
+        assert snapshots.serialize_snapshot(snapshot, auth_user) == {
+            'operation': 'created',
+            'type': 'type',
+            'id': 'id',
+            'user': 'mocked',
+            'data': {'complex': list('object')},
+            'time': datetime(1997, 1, 1),
+        }
 
 
-def test_serializing_tag_category(tag_category_factory):
-    category = tag_category_factory(name='name', color='color')
-    assert snapshots.get_tag_category_snapshot(category) == {
-        'name': 'name',
-        'color': 'color',
-        'default': False,
-    }
-    category.default = True
-    assert snapshots.get_tag_category_snapshot(category) == {
-        'name': 'name',
-        'color': 'color',
-        'default': True,
-    }
-
-
-def test_merging_modification_to_creation(tag_factory, user_factory):
+def test_create(tag_factory, user_factory):
     tag = tag_factory(names=['dummy'])
-    user = user_factory()
-    db.session.add_all([tag, user])
+    db.session.add(tag)
     db.session.flush()
-    snapshots.save_entity_creation(tag, user)
-    tag.names = [db.TagName('changed')]
-    snapshots.save_entity_modification(tag, user)
+    with patch('szurubooru.func.snapshots.get_tag_snapshot'):
+        snapshots.get_tag_snapshot.return_value = 'mocked'
+        snapshots.create(tag, user_factory())
     results = db.session.query(db.Snapshot).all()
     assert len(results) == 1
     assert results[0].operation == db.Snapshot.OPERATION_CREATED
-    assert results[0].data['names'] == ['changed']
+    assert results[0].data == 'mocked'
 
 
-def test_merging_modifications(fake_datetime, tag_factory, user_factory):
-    tag = tag_factory(names=['dummy'])
+def test_modify_saves_non_empty_diffs(post_factory, user_factory):
+    if 'sqlite' in db.sessionmaker.kw['bind'].driver:
+        pytest.xfail(
+            'SQLite doesn\'t support transaction isolation, '
+            'which is required to retrieve original entity')
+    post = post_factory()
+    post.notes = [db.PostNote(polygon=[(0, 0), (0, 1), (1, 1)], text='old')]
     user = user_factory()
-    db.session.add_all([tag, user])
+    db.session.add_all([post, user])
+    db.session.commit()
+    post.source = 'new source'
+    post.notes = [db.PostNote(polygon=[(0, 0), (0, 1), (1, 1)], text='new')]
     db.session.flush()
-    with fake_datetime('13:00:00'):
-        snapshots.save_entity_creation(tag, user)
-    tag.names = [db.TagName('changed')]
-    with fake_datetime('14:00:00'):
-        snapshots.save_entity_modification(tag, user)
-    tag.names = [db.TagName('changed again')]
-    with fake_datetime('14:00:01'):
-        snapshots.save_entity_modification(tag, user)
-    results = db.session.query(db.Snapshot).all()
-    assert len(results) == 2
-    assert results[0].operation == db.Snapshot.OPERATION_CREATED
-    assert results[1].operation == db.Snapshot.OPERATION_MODIFIED
-    assert results[0].data['names'] == ['dummy']
-    assert results[1].data['names'] == ['changed again']
-
-
-def test_not_adding_snapshot_if_data_doesnt_change(
-        fake_datetime, tag_factory, user_factory):
-    tag = tag_factory(names=['dummy'])
-    user = user_factory()
-    db.session.add_all([tag, user])
-    db.session.flush()
-    with fake_datetime('13:00:00'):
-        snapshots.save_entity_creation(tag, user)
-    with fake_datetime('14:00:00'):
-        snapshots.save_entity_modification(tag, user)
+    snapshots.modify(post, user)
     results = db.session.query(db.Snapshot).all()
     assert len(results) == 1
-    assert results[0].operation == db.Snapshot.OPERATION_CREATED
-    assert results[0].data['names'] == ['dummy']
-
-
-def test_not_merging_due_to_time_difference(
-        fake_datetime, tag_factory, user_factory):
-    tag = tag_factory(names=['dummy'])
-    user = user_factory()
-    db.session.add_all([tag, user])
-    db.session.flush()
-    with fake_datetime('13:00:00'):
-        snapshots.save_entity_creation(tag, user)
-    tag.names = [db.TagName('changed')]
-    with fake_datetime('13:10:01'):
-        snapshots.save_entity_modification(tag, user)
-    assert db.session.query(db.Snapshot).count() == 2
-
-
-def test_not_merging_operations_by_different_users(
-        fake_datetime, tag_factory, user_factory):
-    tag = tag_factory(names=['dummy'])
-    user1, user2 = [user_factory(), user_factory()]
-    db.session.add_all([tag, user1, user2])
-    db.session.flush()
-    with fake_datetime('13:00:00'):
-        snapshots.save_entity_creation(tag, user1)
-        tag.names = [db.TagName('changed')]
-        snapshots.save_entity_modification(tag, user2)
-    assert db.session.query(db.Snapshot).count() == 2
-
-
-def test_merging_resets_merging_time_window(
-        fake_datetime, tag_factory, user_factory):
-    tag = tag_factory(names=['dummy'])
-    user = user_factory()
-    db.session.add_all([tag, user])
-    db.session.flush()
-    with fake_datetime('13:00:00'):
-        snapshots.save_entity_creation(tag, user)
-    tag.names = [db.TagName('changed')]
-    with fake_datetime('13:09:59'):
-        snapshots.save_entity_modification(tag, user)
-    tag.names = [db.TagName('changed again')]
-    with fake_datetime('13:19:59'):
-        snapshots.save_entity_modification(tag, user)
-    results = db.session.query(db.Snapshot).all()
-    assert len(results) == 1
-    assert results[0].data['names'] == ['changed again']
-
-
-@pytest.mark.parametrize(
-    'initial_operation',
-    [snapshots.save_entity_creation, snapshots.save_entity_modification])
-def test_merging_deletion_to_modification_or_creation(
-        fake_datetime,
-        tag_factory,
-        tag_category_factory,
-        user_factory,
-        initial_operation):
-    category = tag_category_factory(name='dummy')
-    tag = tag_factory(names=['dummy'], category=category)
-    user = user_factory()
-    db.session.add_all([tag, user])
-    db.session.flush()
-    with fake_datetime('13:00:00'):
-        initial_operation(tag, user)
-    tag.names = [db.TagName('changed')]
-    with fake_datetime('14:00:00'):
-        snapshots.save_entity_modification(tag, user)
-    tag.names = [db.TagName('changed again')]
-    with fake_datetime('14:00:01'):
-        snapshots.save_entity_deletion(tag, user)
-    assert db.session.query(db.Snapshot).count() == 2
-    results = db.session \
-        .query(db.Snapshot) \
-        .order_by(db.Snapshot.snapshot_id.asc()) \
-        .all()
-    assert results[1].operation == db.Snapshot.OPERATION_DELETED
-    assert results[1].data == {
-        'names': ['changed again'],
-        'category': 'dummy',
-        'suggestions': [],
-        'implications': [],
+    assert results[0].data == {
+        'type': 'object change',
+        'value': {
+            'source': {
+                'type': 'primitive change',
+                'old-value': None,
+                'new-value': 'new source',
+            },
+            'notes': {
+                'type': 'list change',
+                'removed': [
+                    {'polygon': [[0, 0], [0, 1], [1, 1]], 'text': 'old'}],
+                'added': [
+                    {'polygon': [[0, 0], [0, 1], [1, 1]], 'text': 'new'}],
+            },
+        },
     }
 
 
-def test_merging_deletion_all_the_way_deletes_all_snapshots(
-        fake_datetime, tag_factory, user_factory):
+def test_modify_doesnt_save_empty_diffs(tag_factory, user_factory):
     tag = tag_factory(names=['dummy'])
     user = user_factory()
     db.session.add_all([tag, user])
-    db.session.flush()
-    with fake_datetime('13:00:00'):
-        snapshots.save_entity_creation(tag, user)
-    tag.names = [db.TagName('changed')]
-    with fake_datetime('13:00:01'):
-        snapshots.save_entity_modification(tag, user)
-    tag.names = [db.TagName('changed again')]
-    with fake_datetime('13:00:02'):
-        snapshots.save_entity_deletion(tag, user)
+    db.session.commit()
+    snapshots.modify(tag, user)
     assert db.session.query(db.Snapshot).count() == 0
 
 
-def test_get_serialized_history(
-        fake_datetime, tag_factory, tag_category_factory, user_factory):
-    category = tag_category_factory(name='dummy')
-    tag = tag_factory(names=['dummy'], category=category)
-    user = user_factory(name='the-user')
-    db.session.add_all([tag, user])
+def test_delete(tag_factory, user_factory):
+    tag = tag_factory(names=['dummy'])
+    db.session.add(tag)
     db.session.flush()
-    with fake_datetime('2016-04-19 13:00:00'):
-        snapshots.save_entity_creation(tag, user)
-    tag.names = [db.TagName('changed')]
+    with patch('szurubooru.func.snapshots.get_tag_snapshot'):
+        snapshots.get_tag_snapshot.return_value = 'mocked'
+        snapshots.delete(tag, user_factory())
+    results = db.session.query(db.Snapshot).all()
+    assert len(results) == 1
+    assert results[0].operation == db.Snapshot.OPERATION_DELETED
+    assert results[0].data == 'mocked'
+
+
+def test_merge(tag_factory, user_factory):
+    source_tag = tag_factory(names=['source'])
+    target_tag = tag_factory(names=['target'])
+    db.session.add_all([source_tag, target_tag])
     db.session.flush()
-    with fake_datetime('2016-04-19 13:10:01'):
-        snapshots.save_entity_modification(tag, user)
-    assert snapshots.get_serialized_history(tag) == [
-        {
-            'operation': 'modified',
-            'time': datetime(2016, 4, 19, 13, 10, 1),
-            'type': 'tag',
-            'id': 'changed',
-            'user': 'the-user',
-            'data': {
-                'names': ['changed'],
-                'category': 'dummy',
-                'suggestions': [],
-                'implications': [],
-            },
-            'earlier-data': {
-                'names': ['dummy'],
-                'category': 'dummy',
-                'suggestions': [],
-                'implications': [],
-            },
-        },
-        {
-            'operation': 'created',
-            'time': datetime(2016, 4, 19, 13, 0, 0),
-            'type': 'tag',
-            'id': 'dummy',
-            'user': 'the-user',
-            'data': {
-                'names': ['dummy'],
-                'category': 'dummy',
-                'suggestions': [],
-                'implications': [],
-            },
-            'earlier-data': None,
-        },
-    ]
+    snapshots.merge(source_tag, target_tag, user_factory())
+    result = db.session.query(db.Snapshot).one()
+    assert result.operation == db.Snapshot.OPERATION_MERGED
+    assert result.data == ['tag', 'target']
diff --git a/server/szurubooru/tests/func/test_tag_categories.py b/server/szurubooru/tests/func/test_tag_categories.py
index 5b67c32b..57aec6c9 100644
--- a/server/szurubooru/tests/func/test_tag_categories.py
+++ b/server/szurubooru/tests/func/test_tag_categories.py
@@ -1,7 +1,7 @@
 from unittest.mock import patch
 import pytest
 from szurubooru import db
-from szurubooru.func import tag_categories, cache, snapshots
+from szurubooru.func import tag_categories, cache
 
 
 @pytest.fixture(autouse=True)
@@ -14,24 +14,21 @@ def test_serialize_category_when_empty():
 
 
 def test_serialize_category(tag_category_factory, tag_factory):
-    with patch('szurubooru.func.snapshots.get_serialized_history'):
-        snapshots.get_serialized_history.return_value = 'snapshot history'
-        category = tag_category_factory(name='name', color='color')
-        category.category_id = 1
-        category.default = True
-        tag1 = tag_factory(category=category)
-        tag2 = tag_factory(category=category)
-        db.session.add_all([category, tag1, tag2])
-        db.session.flush()
-        result = tag_categories.serialize_category(category)
-        assert result == {
-            'name': 'name',
-            'color': 'color',
-            'default': True,
-            'version': 1,
-            'snapshots': 'snapshot history',
-            'usages': 2,
-        }
+    category = tag_category_factory(name='name', color='color')
+    category.category_id = 1
+    category.default = True
+    tag1 = tag_factory(category=category)
+    tag2 = tag_factory(category=category)
+    db.session.add_all([category, tag1, tag2])
+    db.session.flush()
+    result = tag_categories.serialize_category(category)
+    assert result == {
+        'name': 'name',
+        'color': 'color',
+        'default': True,
+        'version': 1,
+        'usages': 2,
+    }
 
 
 def test_create_category_when_first():
diff --git a/server/szurubooru/tests/func/test_tags.py b/server/szurubooru/tests/func/test_tags.py
index dfaa2f4f..f69edef9 100644
--- a/server/szurubooru/tests/func/test_tags.py
+++ b/server/szurubooru/tests/func/test_tags.py
@@ -4,7 +4,7 @@ from unittest.mock import patch
 from datetime import datetime
 import pytest
 from szurubooru import db
-from szurubooru.func import tags, tag_categories, cache, snapshots
+from szurubooru.func import tags, tag_categories, cache
 
 
 @pytest.fixture(autouse=True)
@@ -44,39 +44,36 @@ def test_serialize_tag_when_empty():
 
 
 def test_serialize_tag(post_factory, tag_factory, tag_category_factory):
-    with patch('szurubooru.func.snapshots.get_serialized_history'):
-        snapshots.get_serialized_history.return_value = 'snapshot history'
-        tag = tag_factory(
-            names=['tag1', 'tag2'],
-            category=tag_category_factory(name='cat'))
-        tag.tag_id = 1
-        tag.description = 'description'
-        tag.suggestions = [
-            tag_factory(names=['sug1']), tag_factory(names=['sug2'])]
-        tag.implications = [
-            tag_factory(names=['impl1']), tag_factory(names=['impl2'])]
-        tag.last_edit_time = datetime(1998, 1, 1)
-        post1 = post_factory()
-        post2 = post_factory()
-        post1.tags = [tag]
-        post2.tags = [tag]
-        db.session.add_all([tag, post1, post2])
-        db.session.flush()
-        result = tags.serialize_tag(tag)
-        result['suggestions'].sort()
-        result['implications'].sort()
-        assert result == {
-            'names': ['tag1', 'tag2'],
-            'version': 1,
-            'category': 'cat',
-            'creationTime': datetime(1996, 1, 1, 0, 0),
-            'lastEditTime': datetime(1998, 1, 1, 0, 0),
-            'description': 'description',
-            'suggestions': ['sug1', 'sug2'],
-            'implications': ['impl1', 'impl2'],
-            'usages': 2,
-            'snapshots': 'snapshot history',
-        }
+    tag = tag_factory(
+        names=['tag1', 'tag2'],
+        category=tag_category_factory(name='cat'))
+    tag.tag_id = 1
+    tag.description = 'description'
+    tag.suggestions = [
+        tag_factory(names=['sug1']), tag_factory(names=['sug2'])]
+    tag.implications = [
+        tag_factory(names=['impl1']), tag_factory(names=['impl2'])]
+    tag.last_edit_time = datetime(1998, 1, 1)
+    post1 = post_factory()
+    post2 = post_factory()
+    post1.tags = [tag]
+    post2.tags = [tag]
+    db.session.add_all([tag, post1, post2])
+    db.session.flush()
+    result = tags.serialize_tag(tag)
+    result['suggestions'].sort()
+    result['implications'].sort()
+    assert result == {
+        'names': ['tag1', 'tag2'],
+        'version': 1,
+        'category': 'cat',
+        'creationTime': datetime(1996, 1, 1, 0, 0),
+        'lastEditTime': datetime(1998, 1, 1, 0, 0),
+        'description': 'description',
+        'suggestions': ['sug1', 'sug2'],
+        'implications': ['impl1', 'impl2'],
+        'usages': 2,
+    }
 
 
 def test_export_to_json(