server/tags: fix merging tags

It violated unique constraint on post_tag when a given post was already
tagged with the target tag.

server/szurubooru/func/tags.py

@@ -157,10 +157,19 @@ def delete(source_tag):
     db.session.delete(source_tag)
 
 def merge_tags(source_tag, target_tag):
-    db.session.execute(
-        sqlalchemy.sql.expression.update(db.PostTag) \
-            .where(db.PostTag.tag_id == source_tag.tag_id) \
-            .values(tag_id=target_tag.tag_id))
+    pt1 = sqlalchemy.orm.util.aliased(db.PostTag)
+    pt2 = sqlalchemy.orm.util.aliased(db.PostTag)
+    ids_to_be_tagged = sqlalchemy.sql.expression \
+        .select([pt1.tag_id]) \
+        .where(pt1.tag_id == source_tag.tag_id) \
+        .where(~sqlalchemy.exists() \
+            .where(pt2.post_id == pt1.post_id) \
+            .where(pt2.tag_id == target_tag.tag_id))
+
+    update_stmt = sqlalchemy.sql.expression.update(db.PostTag) \
+        .where(db.PostTag.tag_id.in_(ids_to_be_tagged)) \
+        .values(tag_id=target_tag.tag_id)
+    db.session.execute(update_stmt)
     delete(source_tag)
 
 def create_tag(names, category_name, suggestions, implications):

server/szurubooru/tests/api/test_tag_merging.py

@@ -98,6 +98,28 @@ def test_merging_when_related(test_ctx, fake_datetime):
     assert tags.try_get_tag_by_name('parent').implications == []
     assert tags.try_get_tag_by_name('parent').suggestions == []
 
+def test_merging_when_target_exists(test_ctx, fake_datetime, post_factory):
+    source_tag = test_ctx.tag_factory(names=['source'], category_name='meta')
+    target_tag = test_ctx.tag_factory(names=['target'], category_name='meta')
+    db.session.add_all([source_tag, target_tag])
+    db.session.flush()
+    post1 = post_factory()
+    post1.tags = [source_tag, target_tag]
+    db.session.add_all([post1])
+    db.session.commit()
+    assert source_tag.post_count == 1
+    assert target_tag.post_count == 1
+    with fake_datetime('1997-12-01'):
+        result = test_ctx.api.post(
+            test_ctx.context_factory(
+                input={
+                    'remove': 'source',
+                    'mergeTo': 'target',
+                },
+                user=test_ctx.user_factory(rank=db.User.RANK_REGULAR)))
+    assert tags.try_get_tag_by_name('source') is None
+    assert tags.get_tag_by_name('target').post_count == 1
+
 @pytest.mark.parametrize('input,expected_exception', [
     ({'remove': None}, tags.TagNotFoundError),
     ({'remove': ''}, tags.TagNotFoundError),