How to set taxonomy hierarchical level to 2?

Question

I have created a custom taxonomy and registered it. I want the taxonomy maximum level should me 2. i.e. Parent => child => sub-child. How to set level??

Answer 1

You can't set a level when you create the taxonomy.

The only way to enforce a level is by watching the term creation: Filter create_term; that's the earliest filter that allows us to change parent term ID. For no obvious reasons the filter pre_insert_term is lacking that information … it would probably too easy.

Then change the parent term ID if the current on is already a second level ID.

Sample code, not tested:

add_action( 'create_term', function( $term_id, $tt_id, $taxonomy ) {

    $term = get_term( $term_id );

    // Nothing to do.
    if ( empty( $term->parent ) )
        return;

    $parent = get_term( $term->parent, $taxonomy );

    // This is a second level term, that's okay.
    if ( empty( $parent->parent ) )
        return;

    wp_update_term( $term_id, $taxonomy, [ 'parent' => $parent->term_id ] );
}, 10, 3 );