Automatically add image caption with values from a post parent field?

Question

i have a custom dropdown field on post and i would to get the value from that custom field and insert it on each image caption field uploaded to the current post.

I have been looking for a way to do this and found a couple of examples but not even one work. This morning i found a Wordpress filter that is called attachment_fields_to_save. To my surprise on the codex was an example of almost the same thing i was looking for but i didn't work.

This is the code

function insert_custom_default_caption($post, $attachment) {
if ( substr($post['post_mime_type'], 0, 5) == 'image' ) {
    if ( strlen(trim($post['post_title'])) == 0 ) {
        $post['post_title'] = preg_replace('/\.\w+$/', '', basename($post['guid']));
        $post['errors']['post_title']['errors'][] = __('Empty Title filled from filename.');
    }

    // captions are saved as the post_excerpt, so we check for it before overwriting
    // if no captions were provided by the user, we fill it with our default
    if ( strlen(trim($post['post_excerpt'])) == 0 ) {
        $post['post_excerpt'] = 'default caption';
    }
}

return $post . $attachment;
}

add_filter('attachment_fields_to_save', 'insert_custom_default_caption', 10, 2);

Can anyone help me to see what is wrong with that code?

Answer 1

In the filter, you will need to find the post parent of $post, get the value of the custom field of parent post and then add that value to $post['post_excerpt'] (where the caption is stored):

add_filter('attachment_fields_to_save', 'wpse_insert_custom_caption', 10, 2);
function insert_custom_default_caption($post, $attachment) {

    //Check if the $post is attached to a parent post
    if( $post->post_parent ) {
        //Custom field of the attachment's parent post
        $custom_caption = get_post_meta( $post->post_parent, 'parent_custom_field', true );

        //captions are saved as the post_excerpt
        if ( !empty $custom_caption ) ) {
            $previous_caption = $post['post_excerpt'];
            $post['post_excerpt'] = $previous_caption.$custom_caption;
        }

    }

    return $post;
}