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// Remove visual editor from a custom post type editing page
function remove_editor_init() {
	//remove editor for all registries
	remove_post_type_support('registry', 'editor');

	//remove editor for pages whose parent page id is 321
	if (isset($_GET['post'])) {
		$post_id = $_GET['post'];
	} else if (isset($_POST['post_ID'])) {
		$post_id = $_POST['post_ID'];
	} else {
		return;
	}
	$parents = get_post_ancestors($post_id);
	if ($parents) {
		$topparent_id = $parents[count($parents)-1];
	}
	if ($topparent_id === 321) {
		remove_post_type_support('page', 'editor');
	}
}
add_action('admin_init', 'remove_editor_init');