docs: def add_to_collection(self, request, pk=None):
          """Add Entity to a collection."""
          entity = self.get_object()
  
          # TODO use `self.get_ids` (and elsewhere). Backwards
          # incompatible because raised error's response contains
          # ``detail`` instead of ``error``).
          if 'ids' not in request.data:
              return Response({"error": "`ids` parameter is required"}, status=status.HTTP_400_BAD_REQUEST)
  
          for collection_id in request.data['ids']:
              self._get_collection_for_user(collection_id, request.user)
  
          for collection_id in request.data['ids']:
              entity.collections.add(collection_id)
  
              collection = Collection.objects.get(pk=collection_id)
              for data in entity.data.all():
                  collection.data.add(data)
  
          return Response()