code: function (fn) {
      if (fn.length === 0) throw new Error('Please define your function to accept the command!');
      if (fn.length === 1) {
        var orgFn = fn;
        fn = function(cmd, callback) {
          callback(null, orgFn(cmd));
        };
      }
  
      this.getNewAggregateId = fn;
  
      return this;
    }
docs: Inject idGenerator function for aggregate id that is command aware.
  @param   {Function}  fn The function to be injected.
  @returns {Aggregate}    to be able to chain...