code: public function only(string ...$fields): self
      {
          $forget = $this->fields()->reject(function ($value) use ($fields) {
              return in_array($value, $fields, true);
          });
  
          return $this->forget(...$forget);
      }
docs: Return a new instance that only has the specified attribute/relationship fields.
  
  @param string ...$fields
  @return ResourceObject