doTestRelationshipGet example

  public function testRelationships() {
    if ($this->entity instanceof ConfigEntityInterface) {
      $this->markTestSkipped('Configuration entities cannot have relationships.');
    }

    $request_options = [];
    $request_options[RequestOptions::HEADERS]['Accept'] = 'application/vnd.api+json';
    $request_options = NestedArray::mergeDeep($request_options, $this->getAuthenticationRequestOptions());

    // Test GET.     $this->doTestRelationshipGet($request_options);
    $this->setUpAuthorization('GET');
    $this->doTestRelationshipGet($request_options);

    // Test POST.     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);
    $this->doTestRelationshipMutation($request_options);
    // Grant entity-level edit access.     $this->setUpAuthorization('PATCH');
    $this->doTestRelationshipMutation($request_options);
    // Field edit access is still forbidden, grant it.     $this->grantPermissionsToTestedRole([