Send an email when custom post type category is changed

Question

This is the first time I've attempted to write an action hook.

My CPT has 4 categories. I'm trying to make WP automatically sends an email notification when the category is changed.

By change I mean: Let's say the post has a category of "cat1". The admin unselects cat1 and selects "cat2" and then clicks the "Update" button. I am not talking about overwriting or appending new categories to the list.

My code thus far:

add_action('set_object_terms','email_when_category_changes');

function email_when_category_changes(){
       wp_mail( '[email protected]', 'Update on CI board', 'the category changed' );
}

The idea here being that the author will get an email notifying them that their post has been changed from cat1 to cat2.

The code above works in the sense that it sends an email when I click "Update". However, the problem is that it sends an email no matter what I do. Anytime I click "Update", whether anything changed or not, it sends the email.

I thought set_object_terms would only fire if the category was changed. Obviously I'm missing an important concept here. Can anyone help?

Answer 1

Use the hooks added_term_relationship and deleted_term_relationships. These only fire when the relationship changes, as opposed to set_object_terms which always fires:

function wpse_181090_object_terms_updated( $object_id ) {
    static $did = array(); // This function might fire multiple times for the same object, ensure it only runs once

    if ( ! isset( $did[ $object_id ] ) ) {
        $did[ $object_id ] = true;

        // Your notification code
    }
}

add_action( 'deleted_term_relationships', 'wpse_181090_object_terms_updated' );
add_action( 'added_term_relationship',    'wpse_181090_object_terms_updated' );