Saving attachment custom fields

Question

I want to establish a relationship between individual post attachments and Users.
So far I've got my users listing just fine, but it isn't saving/updating.

add_filter("attachment_fields_to_edit", "my_image_attachment_fields_to_edit", null, 2);

function my_image_attachment_fields_to_edit($form_fields, $post) {

    $form_fields["belongs_to"]["label"] = 'Belongs to';  
    $form_fields["belongs_to"]["input"] = "html";  
    $form_fields["belongs_to"]["html"] = '';

    $wp_user_query = new WP_User_Query( array( 'orderby' => 'display_name' ) );
    $authors = $wp_user_query->get_results();
if (!empty($authors)) { 
    foreach ($authors as $author) {
        $author_info = get_userdata($author->ID);                       
        $form_fields["belongs_to"]["html"] .= "
        <div style='float:left;width:50%'>
            <input type='checkbox' value='$author_info->ID' name='attachments[{$post->ID}][belongs_to][]' id='belongs_to_user-$author_info->ID' />
            <label for='belongs_to_user-$author_info->ID'>$author_info->first_name $author_info->last_name</label>
        </div>";
    }
}

return $form_fields;
}


add_filter("attachment_fields_to_save", "my_image_attachment_fields_to_save", null, 2);
function my_image_attachment_fields_to_save($post, $attachment) {
if( isset($attachment['my_field']) ){
    // update_post_meta(postID, meta_key, meta_value);
    update_post_meta($post['ID'], 'belongs_to', $attachment['belongs_to']);
}
return $post;
}

I was trying to populate an array with user-IDs, but perhaps that's not the best way to handle it. Any ideas? Thanks in advance!

Answer 1

Got it:

add_filter("attachment_fields_to_edit", "my_image_attachment_fields_to_edit", null, 2);
function my_image_attachment_fields_to_edit($form_fields, $post) {

    $form_fields["OwnersHeading"]["tr"] = "
    <tr>
        <td colspan='2' style='font-size:16px;padding-left:15px'>Tag Users:</td>
    </tr>";

    // get list of users
    $wp_user_query = new WP_User_Query( array( 'orderby' => 'display_name' ) );
    $authors = $wp_user_query->get_results();

    if (!empty($authors)) { 
        foreach ($authors as $author) {

            $belongs_to_value = (bool)get_post_meta( $post->ID, "_owner_$author->ID", true );

            // create the field
            $form_fields["owner_$author->ID"] = array(
                'input' => 'html',
                'html'  => '
                    <input type="checkbox" id="attachments-' . $post->ID . '-owner'.$author->ID.'" name="attachments[' . $post->ID . '][owner_'.$author->ID.']" value="1"' . ( $belongs_to_value ? ' checked="checked"' : '' ) . ' />
                    <label for="attachments-' . $post->ID . '-owner'.$author->ID.'">'.$author_info->first_name.' '.$author_info->last_name.'</label>',
            );

        }
    }

    return $form_fields;
}


add_filter("attachment_fields_to_save", "my_image_attachment_fields_to_save", null, 2);
function my_image_attachment_fields_to_save($post, $attachment) {

    // get list of users
    $wp_user_query = new WP_User_Query( array( 'orderby' => 'display_name' ) );
    $authors = $wp_user_query->get_results();

    if (!empty($authors)) {
        foreach ($authors as $author) {
            update_post_meta($post['ID'], '_owner_'.$author->ID.'', $attachment['owner_'.$author->ID.'']);      
        }
    }

    return $post;
}