code: function() {
      if (this._isSerializing){ return this.cid; }
      var val = this.value();
      this._isSerializing = true;
      var json = (val && _.isFunction(val.toJSON)) ? val.toJSON() : val;
      this._isSerializing = false;
      return json;
    }
docs: Cyclic dependancy safe toJSON method.