docs: function(event, entity, collection, options) {
        if (entity) {
          if ((event === 'add' || event === 'remove') && collection !== this) return;
          if (event === 'destroy') this.remove(entity, options);
          if (event === 'change') {
            var prevId = this.entityId(entity.previousAttributes());
            var id = this.entityId(entity.attributes);
            if (prevId !== id) {
              if (prevId != null) delete this._byId[prevId];
              if (id != null) this._byId[id] = entity;
            }
          }
        }
        this.trigger.apply(this, arguments);
      }