diff --git a/server/szurubooru/db/comment.py b/server/szurubooru/db/comment.py
index bbb8b189..bf325859 100644
--- a/server/szurubooru/db/comment.py
+++ b/server/szurubooru/db/comment.py
@@ -8,11 +8,16 @@ class CommentScore(Base):
     __tablename__ = 'comment_score'
 
     comment_id = Column(
-        'comment_id', Integer, ForeignKey('comment.id'), primary_key=True)
+        'comment_id',
+        Integer,
+        ForeignKey('comment.id'),
+        nullable=False,
+        primary_key=True)
     user_id = Column(
         'user_id',
         Integer,
         ForeignKey('user.id'),
+        nullable=False,
         primary_key=True,
         index=True)
     time = Column('time', DateTime, nullable=False)
@@ -29,8 +34,9 @@ class Comment(Base):
 
     comment_id = Column('id', Integer, primary_key=True)
     post_id = Column(
-        'post_id', Integer, ForeignKey('post.id'), index=True, nullable=False)
-    user_id = Column('user_id', Integer, ForeignKey('user.id'), index=True)
+        'post_id', Integer, ForeignKey('post.id'), nullable=False, index=True)
+    user_id = Column(
+        'user_id', Integer, ForeignKey('user.id'), nullable=True, index=True)
     version = Column('version', Integer, default=1, nullable=False)
     creation_time = Column('creation_time', DateTime, nullable=False)
     last_edit_time = Column('last_edit_time', DateTime)
diff --git a/server/szurubooru/db/post.py b/server/szurubooru/db/post.py
index d68b4e38..65942d37 100644
--- a/server/szurubooru/db/post.py
+++ b/server/szurubooru/db/post.py
@@ -31,12 +31,14 @@ class PostScore(Base):
         Integer,
         ForeignKey('post.id'),
         primary_key=True,
+        nullable=False,
         index=True)
     user_id = Column(
         'user_id',
         Integer,
         ForeignKey('user.id'),
         primary_key=True,
+        nullable=False,
         index=True)
     time = Column('time', DateTime, nullable=False)
     score = Column('score', Integer, nullable=False)
@@ -55,12 +57,14 @@ class PostFavorite(Base):
         Integer,
         ForeignKey('post.id'),
         primary_key=True,
+        nullable=False,
         index=True)
     user_id = Column(
         'user_id',
         Integer,
         ForeignKey('user.id'),
         primary_key=True,
+        nullable=False,
         index=True)
     time = Column('time', DateTime, nullable=False)
 
@@ -90,12 +94,14 @@ class PostRelation(Base):
         Integer,
         ForeignKey('post.id'),
         primary_key=True,
+        nullable=False,
         index=True)
     child_id = Column(
         'child_id',
         Integer,
         ForeignKey('post.id'),
         primary_key=True,
+        nullable=False,
         index=True)
 
     def __init__(self, parent_id, child_id):
@@ -111,9 +117,15 @@ class PostTag(Base):
         Integer,
         ForeignKey('post.id'),
         primary_key=True,
+        nullable=False,
         index=True)
     tag_id = Column(
-        'tag_id', Integer, ForeignKey('tag.id'), primary_key=True, index=True)
+        'tag_id',
+        Integer,
+        ForeignKey('tag.id'),
+        primary_key=True,
+        nullable=False,
+        index=True)
 
     def __init__(self, post_id, tag_id):
         self.post_id = post_id
diff --git a/server/szurubooru/db/snapshot.py b/server/szurubooru/db/snapshot.py
index f0a497bd..45f1077a 100644
--- a/server/szurubooru/db/snapshot.py
+++ b/server/szurubooru/db/snapshot.py
@@ -19,7 +19,10 @@ class Snapshot(Base):
     resource_repr = Column('resource_repr', Unicode(64), nullable=False)
     operation = Column('operation', Unicode(16), nullable=False)
     user_id = Column(
-        'user_id', Integer, ForeignKey('user.id', ondelete='set null'))
+        'user_id',
+        Integer,
+        ForeignKey('user.id', ondelete='set null'),
+        nullable=True)
     data = Column('data', PickleType)
 
     user = relationship('User')
diff --git a/server/szurubooru/db/tag.py b/server/szurubooru/db/tag.py
index 5dae8c8b..94b1c2c5 100644
--- a/server/szurubooru/db/tag.py
+++ b/server/szurubooru/db/tag.py
@@ -13,12 +13,16 @@ class TagSuggestion(Base):
         'parent_id',
         Integer,
         ForeignKey('tag.id'),
-        primary_key=True, index=True)
+        nullable=False,
+        primary_key=True,
+        index=True)
     child_id = Column(
         'child_id',
         Integer,
         ForeignKey('tag.id'),
-        primary_key=True, index=True)
+        nullable=False,
+        primary_key=True,
+        index=True)
 
     def __init__(self, parent_id, child_id):
         self.parent_id = parent_id
@@ -32,12 +36,14 @@ class TagImplication(Base):
         'parent_id',
         Integer,
         ForeignKey('tag.id'),
+        nullable=False,
         primary_key=True,
         index=True)
     child_id = Column(
         'child_id',
         Integer,
         ForeignKey('tag.id'),
+        nullable=False,
         primary_key=True,
         index=True)