code: function (cmd, callback) {
      var getNewIdFn = this.eventStore.getNewId.bind(this.eventStore);
      if (this.aggregate && this.aggregate.getNewAggregateId) {
        getNewIdFn = this.aggregate.getNewAggregateId.bind(this.aggregate);
      } else if (this.getNewAggregateIdFn) {
        getNewIdFn = this.getNewAggregateIdFn.bind(this);
      }
  
      if (getNewIdFn.length === 2) {
        return getNewIdFn(cmd, callback);
      }
  
      getNewIdFn(callback);
    }
docs: IdGenerator function for aggregate id.
  @param {Function} callback The function, that will be called when this action is completed.
  `function(err, newId){}`