protected function getIncluded(ElementInterface $element, $includeParent = false)
    {
        $included = [];

        foreach ($element->getResources() as $resource) {
            if ($resource->isIdentifier()) {
                continue;
            }

            if ($includeParent) {
                $included = $this->mergeResource($included, $resource);
            } else {
                $type = $resource->getType();
                $id = $resource->getId();
            }

            foreach ($resource->getUnfilteredRelationships() as $relationship) {
                $includedElement = $relationship->getData();

                if (! $includedElement instanceof ElementInterface) {
                    continue;
                }

                foreach ($this->getIncluded($includedElement, true) as $child) {
                    // If this resource is the same as the top-level "data"
                    // resource, then we don't want it to show up again in the
                    // "included" array.
                    if (! $includeParent && $child->getType() === $type && $child->getId() === $id) {
                        continue;
                    }

                    $included = $this->mergeResource($included, $child);
                }
            }
        }

        $flattened = [];

        array_walk_recursive($included, function ($a) use (&$flattened) {
            $flattened[] = $a;
        });

        return $flattened;
    }