docs: def add_data(self, request, pk=None):
          """Add data to Entity and it's collection."""
          # add data to entity
          resp = super().add_data(request, pk)
  
          # add data to collections in which entity is
          entity = self.get_object()
          for collection in entity.collections.all():
              collection.data.add(*request.data['ids'])
  
          return resp