docs: function (authenticationRequest, callback) {
              // create query to fetch the user
              var query = this._createQueryForUser(authenticationRequest.get(this.$.usernameField));
              var collection = this.$.dataSource.createCollection(this.$collectionClass).query(query);
  
              collection.fetch({limit: 1}, function (err, users) {
                  var user;
                  if (!err && users.size()) {
                      user = users.at(0);
                      user.fetch(null, callback);
                  } else {
                      callback(err, null);
                  }
              });
          }