If meta key exists in get posts function otherwise create it

Question

The following code fetches posts with the meta key home telephone numbers.

$metas = ''; 
    $args = array(
    'numberposts' => 1,
    'post_type' => 'electors',
    'orderby' => 'post_date',
    'order' => 'ASC',
    'meta_query' => array(
        array(
            'key' => 'wpcf-home_telephone_number',
            'value' => '0',
            'compare' => '>'
        )
     ) 
    );


$canvassing = get_posts( $args );

I want to add another meta to look for. The problem is that this meta may not exist on all posts. I want to get posts that either don't have the meta key or don't have a value for it.

array(
            'key' => 'wpcf-success',

            'value' => ''
        )

This code will only get posts that have the key without a value. I also want posts that do not have this key at all.

I tired this add_post_meta($post->ID, 'wpcf-success', '', true); but since we have not fetched the posts there is no post ID yet.

Answer 1

This "not exists" functionality isn't possible using a regular meta query. It's being worked on, but it looks like it's been shuffled to a future release (see this trac ticket for more insight).

In the meantime, you have to run a custom SQL statement using $wpdb, as in DeadMedic's answer that Milo linked to in the comments.