仅在wordpress中匹配子类别进行查询

Question

我有一个名为“项目”的类别。我想查询各种子类别，只要它们匹配。我该怎么做呢？下面的代码只查询主要类别“项目”的所有子项。

if (is_single()) {
    $cats = get_categories('child_of=31');
    $cat = $cats[0];

} else {
    $cat = get_category(get_query_var('cat'));
}
$cat_slug = $cat - > slug;
$work_query = new WP_Query(array(
    'post_type' => 'work',
    'category__in' => array($cat - > term_id),
    'posts_per_page' => 20,
    'orderby' => 'date',
    'order' => 'DESC',
    'paged' => $paged,
    'post__not_in' => array(get_the_ID()),
));