public function testToIdentifierReturnsArrayOfResourceIdentifiers() { $serializer = new PostSerializer3(); $post1 = (object) ['id' => 1]; $post2 = (object) ['id' => 2]; $collection = new Collection([$post1, $post2], $serializer); $resource1 = new Resource($post1, $serializer); $resource2 = new Resource($post2, $serializer); $this->assertEquals([$resource1->toIdentifier(), $resource2->toIdentifier()], $collection->toIdentifier()); }