protected function addCategory(CategoryInterface $category)
{
$key = $category->getKey();
if (isset($this->categories[$key])) {
// combine categories
$data = Category::toArray($this->categories[$key]);
$data = array_merge($data, Category::toArray($category));
$category = $this->createCategory($data);
}
$this->categories[$key] = $category;
}
