code: protected function methodForRelation($field)
      {
          if (method_exists($this, $field)) {
              return $field;
          }
  
          $method = Str::camelize($field);
  
          return method_exists($this, $method) ? $method : null;
      }
docs: Get the method name on this adapter for the supplied JSON API field.
  
  By default we expect the developer to be following the PSR1 standard,
  so the method name on the adapter should use camel case.
  
  However, some developers may prefer to use the actual JSON API field
  name. E.g. they could use `user_history` as the JSON API field name
  and the method name.
  
  Therefore we return the field name if it exactly exists on the adapter,
  otherwise we camelize it.
  
  A developer can use completely different logic by overloading this
  method.
  
  @param string $field
  the JSON API field name.
  @return string|null
  the adapter's method name, or null if none is implemented.