• kyon147

• July 12, 2015 at 9:40 pm

I have managed to work it out here is the answer if anyone needs it. This would go in your function.php file

// retrieves the attachment ID from the file URL
function get_image_id($image_url) {
    global $wpdb;
    $attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url ));
    return $attachment[0];
}

//Set the featured image on save of the post
add_action('save_post', 'set_featured_image_from_image');

//set featured image as main image.
function set_featured_image_from_image()
{
        $has_thumbnail = get_the_post_thumbnail($post->ID);
        
        //If there is not featured image get the other image.
        if (!$has_thumbnail) {
        //get the url of the ACF image and get the ID of that image
            $image_url = get_field('main_image');
            $image_id = get_image_id($image_url);

            if ($image_id) {
                set_post_thumbnail($post->ID, $image_id);
            }
        }
    
}

I am outputting just the url from the image so I had to reverse get the ID. Those who are using the full object should be able to get the ID more easily.