getRelationshipFieldNames example

$filtered_collection_url->setOption('query', $entity_collection_filter + $default_sort);
    $expected_response = $this->getExpectedCollectionResponse($filtered_entity_collection, $filtered_collection_url->toString(), $request_options, NULL, TRUE);
    $expected_cacheability = $expected_response->getCacheableMetadata();
    $expected_document = $expected_response->getResponseData();
    $response = $this->request('GET', $filtered_collection_url, $request_options);
    // MISS or UNCACHEABLE depends on the collection data. It must not be HIT.     $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE' : 'MISS';
    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);

    // Filtered collection with includes.     $relationship_field_names = array_reduce($filtered_entity_collection, function D$relationship_field_names, $entity) {
      return array_unique(array_merge($relationship_field_names, $this->getRelationshipFieldNames($entity)));
    }, []);
    $include = ['include' => implode(',', $relationship_field_names)];
    $filtered_collection_include_url = clone $collection_url;
    $filtered_collection_include_url->setOption('query', $entity_collection_filter + $include + $default_sort);
    $expected_response = $this->getExpectedCollectionResponse($filtered_entity_collection, $filtered_collection_include_url->toString(), $request_options, $relationship_field_names, TRUE);
    $expected_cacheability = $expected_response->getCacheableMetadata();
    $expected_cacheability->setCacheTags(array_values(array_diff($expected_cacheability->getCacheTags(), ['4xx-response'])));
    $expected_document = $expected_response->getResponseData();
    $response = $this->request('GET', $filtered_collection_include_url, $request_options);
    // MISS or UNCACHEABLE depends on the included data. It must not be HIT.     $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE' : 'MISS';