Set default Custom Post Meta Value

Question

I've tried almost anything and just can't get this to work.

I have a custom post type with a custom meta field and I want a default value to be added on when the Publish/Update/Save button is pressed.

I tried all kind of hooks wp_insert_post, save_post, edit_post..

add_action('save_post_post_type', 'set_default_meta', 10, 3);

And my function is:

function set_default_meta( $post_ID ) {

    $meta_value = get_post_meta($post_ID, 'meta_value', true);

    if ( $meta_value == '' && !wp_is_post_revision($post_ID) ) {
        add_post_meta( $post_ID, 'meta_value', 0, true );
    }
}

When I publish the post, the value is not saved. However whenI click View Post, the value is immediately saved.

Any help will be really appreciated!

Answer 1

Use update_post_meta(), because if the meta key already exists add_post_meta() will do nothing if $unique is true and otherwise won't update neither, while update_post_meta(), well, it will update the value of a existing field or create it, if it doesn't exist yet.