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
Add data to Entity and it's collection.