code: def update(self, instance):
          
          assert isinstance(instance, self.model_klass)
          if instance.db_id:
              query = {'_id': ObjectId(instance.db_id)}
          else:
              query = build_db_query(self.primary_key, instance.key)
          self.ds.update(self.collection_name, query, instance)
          return instance.db_id
docs: this is an upsert method: replaces or creates the DB representation of the model instance