code: function(model, options) {
        delete this._byId[model.cid];
        var id = this.modelId(model.attributes);
        if (id != null) delete this._byId[id];
        if (this === model.collection) delete model.collection;
        model.off('all', this._onModelEvent, this);
      }
docs: Internal method to sever a model's ties to a collection.