Tobscure\JsonApi\Collection::toIdentifier PHP Method

    public function toIdentifier()
    {
        return array_map(function (Resource $resource) {
            return $resource->toIdentifier();
        }, $this->resources);
    }

 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());
 }