server/posts: fix post relations

Trying to relate post to itself resulted in 500 ISE.

server/szurubooru/func/posts.py

@@ -377,6 +377,8 @@ def update_post_relations(post, new_post_ids):
         .all()
     if len(new_posts) != len(new_post_ids):
         raise InvalidPostRelationError('One of relations does not exist.')
+    if post.post_id in new_post_ids:
+        raise InvalidPostRelationError('Post cannot relate to itself.')
 
     relations_to_del = [p for p in old_posts if p.post_id not in new_post_ids]
     relations_to_add = [p for p in new_posts if p.post_id not in old_post_ids]

server/szurubooru/tests/func/test_posts.py

@@ -541,6 +541,14 @@ def test_update_post_relations_with_nonexisting_posts():
         posts.update_post_relations(post, [100])
 
 
+def test_update_post_relations_with_itself(post_factory):
+    post = post_factory()
+    db.session.add(post)
+    db.session.flush()
+    with pytest.raises(posts.InvalidPostRelationError):
+        posts.update_post_relations(post, [post.post_id])
+
+
 def test_update_post_notes():
     post = db.Post()
     posts.update_post_notes(