code: public function pointerForRelationship(string $key, string $default = '/'): string
      {
          $field = collect(explode('.', $key))->first();
  
          if (!$this->isRelationship($field)) {
              throw new \InvalidArgumentException("Field {$field} is not a relationship.");
          }
  
          $pointer = $this->pointer($key);
  
          return Str::after($pointer, "relationships/{$field}") ?: $default;
      }
docs: Convert a validation key to a JSON pointer for a relationship object within the resource.
  
  @param string $key
  @param string $default
  @return string