code: function(userObjectId) {
          if (!userObjectId || !_.isString(userObjectId)) {
            throw new Error('Invalid user object id.');
          }
          var query = new AV.FriendShipQuery('_Follower');
          query._friendshipTag = 'follower';
          query.equalTo(
            'user',
            AV.Object.createWithoutData('_User', userObjectId)
          );
          return query;
        }
docs: Create a follower query for special user to query the user's followers.
  @param {String} userObjectId The user object id.
  @return {AV.FriendShipQuery}
  @since 0.3.0