code: function (fn) {
      if (!fn || !_.isFunction(fn)) {
        var err = new Error('Please pass a valid function!');
        debug(err);
        throw err;
      }
  
      if (fn.length === 1) {
        this.getNewId = fn;
        return this;
      }
  
      this.getNewId = function (callback) {
        callback(null, fn());
      };
  
      return this;
    }
docs: Inject idGenerator function.
  @param   {Function}  fn The function to be injected.
  @returns {Domain}       to be able to chain...