code: function (fn) {
      if (fn.length === 0) {
        fn = _.wrap(fn, function(func, callback) {
          callback(null, func());
        });
      }
  
      this.getNewId = fn;
  
      return this;
    }
docs: Inject idGenerator function.
  @param   {Function}  fn The function to be injected.
  @returns {Aggregate}    to be able to chain...