Is there better way to do this without duplicating queries?

Question

I'm building a plugin to show message based on which category user is browsing. For that I want to get all categories and I'm getting all categories by running:

$all_categories = get_categories(array('hide_empty' => false ));
$all_category_ids = array();
foreach($all_categories as $category)
{       
    array_push($all_category_ids, $category->term_id);    
}

Then I'm getting sub categories which I want to hide using:

$exclude_categories = get_categories(array( 'child_of' => 55, 'hide_empty' => false ));
$exclude_category_ids = array();    
foreach($exclude_categories as $ex_cat)
{
    array_push($exclude_category_ids, $ex_cat->term_id); 
}

Then I'm getting the difference by:

$to_show = array_diff($all_category_ids,$exclude_category_ids)

This gets the job done but query monitor plugin showing that there is duplicate queries which i have by running get_categories() twice.

Is there better way of doing this?

Answer 1

Since your first get_categories call will load all terms into memory, you can just loop over 'em and check if they have an ancestor with ID 55, and WordPress won't ever hit the db again:

$terms = get_terms([
     'taxonomy'   => 'category',
     'hide_empty' => false,
]);

foreach ( $terms as $k => $term ) {
    if ( in_array( 55, get_ancestors( $term->term_id, 'category', 'taxonomy' ) ) )
        unset( $terms[ $k ] );
}