Automatically assign parent taxonomy terms to posts

How it works:

Using hook set_object_terms anytime the categories are updated, this function will check whether they belong to a parent category and select the parent categories as well (and parent of parent, etc).

Note:

Do not use save_post to update categories.. because when you save a post where you only update the categories.. the save_post hook is not called.

add_action( 'set_object_terms', 'auto_set_parent_terms', 9999, 6 );
/**
 * Automatically set/assign parent taxonomy terms to posts
 *
 * This function will automatically set parent taxonomy terms whenever terms are set on a post,
 * with the option to configure specific post types, and/or taxonomies.
 *
 *
 * @param int    $object_id  Object ID.
 * @param array  $terms      An array of object terms.
 * @param array  $tt_ids     An array of term taxonomy IDs.
 * @param string $taxonomy   Taxonomy slug.
 * @param bool   $append     Whether to append new terms to the old terms.
 * @param array  $old_tt_ids Old array of term taxonomy IDs.
 */
function auto_set_parent_terms( $object_id, $terms, $tt_ids, $taxonomy, $append, $old_tt_ids ) {

    /**
     * We only want to move forward if there are taxonomies to set
     */
    if( empty( $tt_ids ) ) return FALSE;

    /**
     * Set specific post types to only set parents on.  Set $post_types = FALSE to set parents for ALL post types.
     */
    $post_types = array( 'post' );
    if( $post_types !== FALSE && ! in_array( get_post_type( $object_id ), $post_types ) ) return FALSE;

    /**
     * Set specific post types to only set parents on.  Set $post_types = FALSE to set parents for ALL post types.
     */
    $tax_types = array( 'category' );
    if( $tax_types !== FALSE && ! in_array( get_post_type( $object_id ), $post_types ) ) return FALSE;

    foreach( $tt_ids as $tt_id ) {

        $parent = wp_get_term_taxonomy_parent_id( $tt_id, $taxonomy );

        if( $parent ) {
            wp_set_post_terms( $object_id, array($parent), $taxonomy, TRUE );
        }

    }

}

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