code: function field(query) {
      var queryDefinition;
      if (query instanceof Function) {
          queryDefinition = query();
      }
      else {
          queryDefinition = query;
      }
      var customField = queryDefinition.select;
      customField = customField.addSubQuery(queryDefinition);
      // Field query cannot be joined to any other query so don't have set the positional fields
      return customField;
  }
docs: Sub-queries in select clause
  @param query
  @returns {IQF}