code: function (identifier) {
              var self = this;
  
              return this.findByIdentifier(identifier).then(function (record) {
                  if (!record) {
                      throw new Errors.NotFoundError(
                          'Could not find %s with identifier %s'.format(
                              pluralize.singular(self.identity),
                              identifier
                          ), {
                              identifier: identifier,
                              collection: self.identity
                          });
                  }
  
                  return record;
              });
          }
docs: Look up by identifier - either the "id" or in the identifiers array.
  Returns a waterline (bluebird) promise that will resolve the promise.
  Rejects the promise if the document was not found.
  
  @param {string} identifier
  @returns {Deferred}