docs: function (key) {
          var ret = emberA();
          var seen = new _emberMetalEmpty_object.default();
  
          this.forEach(function (item) {
            var guid = _emberMetalUtils.guidFor(_emberMetalProperty_get.get(item, key));
            if (!(guid in seen)) {
              seen[guid] = true;
              ret.push(item);
            }
          });
  
          return ret;
        }