This block is basically doing what `self._datasource_ex` will do immediately after it, only for custom filters. By moving this logic to `_datasource_ex` you would enable the user filter for PATCH and DELETE too (an authorized user could delete an item he didn't save as long as he knows the id). I didn't try but I believe that by putting `user: request.authorization.username` in a custom filter (which is already supported) you would not need this addition at all (although custom filters need to be set for every resource, not very handy).