code: function (req, res, options) {
      if (!options) {
        options = {};
      }
      options.operation = 'AGGREGATE';
      options.res = res;
      exports.aggregate.before(req, res, options);
      vulpejs.models.aggregate({
        model: req.params.model,
        aggregate: req.params.aggregate,
        callback: {
          success: function (results) {
            if (options && options.callback) {
              vulpejs.utils.execute(options.callback, results);
            } else {
              exports.aggregate.after(req, res, results);
              res.json({
                items: results,
              });
            }
          },
          error: function (error) {
            vulpejs.log.error('AGGREGATE', error);
            res.status(500).end();
          },
        },
      });
    }
docs: Aggregate object/objects in database an execute callback.
  @param {Object} req     Request
  @param {Object} res     Response
  @param {Object} options {model, group, callback}