code: function(instance, objects) {
      var self = this;
      var after = this.afterAddingObjects.bind(this, instance, objects);
      return self._updateForeignKeys(instance, objects, instance.id).tap(after);
    }
docs: Perform the necessary updates to add objects for this relation.
  
  This method invalidates the {@link HasMany#objectsQuery}.
  
  Mixins can override {@link HasMany#executeAdd} to change the way updates
  are performed when related objects are added. This is the default
  implementation.
  
  @method
  @protected
  @param {Model} instance The model instance on which to operate.
  @param {Array} objects The objects to add to the relationship.