code: function attrDescriptor(fieldName) {
      return {
          get() {
              return this._fields[fieldName];
          },
  
          set(value) {
              return this.set(fieldName, value);
          },
  
          enumerable: true,
          configurable: true,
      };
  }
docs: The functions in this file return custom JS property descriptors
  that are supposed to be assigned to Model fields.
  
  Some include the logic to look up models using foreign keys and
  to add or remove relationships between models.
  
  @module descriptors
   
  Defines a basic non-key attribute.
  @param  {string} fieldName - the name of the field the descriptor will be assigned to.