api.db: fix Database.update method

Database `update` method was breaking while creating a
`User` model with `is_superuser=1` field value.
Fix the method by separating `Node` specific logic from
other model updates based on below facts:
- `obj.update()` will trigger `BeanieBaseUser.update()` for `User`
model and not `DatabaseModel.update()`
- `User` model doesn't have `parent` field

Signed-off-by: Jeny Sadadia <[email protected]>

api/db.py

@@ -278,9 +278,10 @@ async def update(self, obj):
         if obj.id is None:
             raise ValueError("Cannot update object with no id")
         col = self._get_collection(obj.__class__)
-        obj.update()
-        if obj.parent == obj.id:
-            raise ValueError("Parent cannot be the same as the object")
+        if obj.__class__ == Node:
+            obj.update()
+            if obj.parent == obj.id:
+                raise ValueError("Parent cannot be the same as the object")
         res = await col.replace_one(
             {'_id': ObjectId(obj.id)}, obj.dict(by_alias=True)
         )