code: public function queryToOne($relation, EncodingParametersInterface $parameters)
      {
          return $this->queryOne(
              $relation->newQuery(),
              $this->getQueryParameters($parameters)
          );
      }
docs: Query the resource when it appears in a to-one relation of a parent resource.
  
  For example, a request to `/posts/1/author` will invoke this method on the
  user adapter when the author relation returns a `users` resource.
  
  @param Relations\BelongsTo|Relations\HasOne|Builder $relation
  @param EncodingParametersInterface $parameters
  @return mixed