How to avoid overwriting meta with empty metaboxes? [closed]

Question

I have a custom post type with about 20 different metaboxes. What I would like to do is be able to hit "New post" for this type, enter the post title, fill in an unique identifier on one of the metafields (like a Freebase ID), and then go out and query Freebase, automatically bringing in the data for the other 19 metas.

The going out and getting data works just fine, and when I tested it using different meta keys, it works like a charm. But when I use update_post_meta to set the values to the proper keys (ie the ones actually used by my metaboxes), the metadata gets overwritten by the empty metaboxes in the Edit Post form.

I had been hooking the function that gets the external metadata into save_post. Is there a better method? Better place to hook into? Is there an action somewhere between saving the data from the form and the redirect to the edit page where I could perform the update_post_meta function?

Thanks in advance.

UPDATE: Here's the code - shortened, as I figured showing four of the metas was sufficient.

Again, this code works perfectly IF I save the metadata to keys not being used by the theme (ie if I append the word "Test" to each metakey below, it works fine. That's why I'm assuming the metas are being overwritten by the blank meta boxes in the post edit form.)

<?php
function get_album_data($post_id) {
// after the user fills in the Title and ID and hits "Save Draft"
// this function goes out and gets every piece of information available about this album
global $wpdb;
$slug = 'album';

// check whether anything should be done 
$_POST += array("{$slug}_edit_nonce" => '');
if ( $slug != $_POST['post_type'] ) {
    return;
}
if ( !current_user_can( 'publish_posts', $post_id ) ) {
    return;
}
// set a flag so this only runs once
$initial_data_imported = get_post_meta($post_id, 'initial_data_imported', $single);
if ( !empty( $initial_data_imported ) ) {
    return;
}

/* Request passes all checks; update the post's metadata */
if (isset($_REQUEST['Album_ID'])) {
    $album_id = $_REQUEST['Album_ID'];

    // curl request info goes here

    $album_artist = $search_results->album->artist;
    $album_producer = $search_results->album->producer;
    $album_label = $search_results->album->label;
    update_post_meta ($post_id, 'Artist', $album_artist);
    update_post_meta ($post_id, 'Producer', $album_producer);
    update_post_meta ($post_id, 'Label', $album_label);
    update_post_meta( $post_id, 'initial_data_imported', true );
    }
}

add_action( 'save_post', 'get_album_data' );
?>

Answer 1

If I understand correctly, the theme you have has metaboxes built in for your album post type, and the theme handles saving the post meta contained in these boxes. However, you would like to use one field to auto-populate the rest of the meta.

(Note: you seem to be using the terms Custom Field and metaboxes interchangeably. They are different. I'm assuming that you using metaboxes.).

The set up you have should work. But your save_post callback is (probably) being called before the theme's. Thus you update the post meta with the remotely retrieved data, but then afterwards the theme updates the post meta with data it received from the form (i.e. an empty string).

The trick is to ensure your callback is called after theme's. The priority for the theme's callback is probably the default - 10. But you can't be sure unless you inspect the code. Try setting your callback's priority to 20:

add_action( 'save_post', 'get_album_data', 20 );

Other things I should point out:

• Prefix your function names with something unique e.g.: wpse56728_get_album_data. This reduces the risk of name clash
• Check that the passed post isn't a revision: wp_is_post_revision($post_id) and just return from your function early if it is.
• Ideally you should check the metabox's nonce (it should have one) prior to processing.
• Don't use curl. See this on HTTP requests with WordPress.

Answer 2

It turns out my theme had a function hooked into save_post that was overwriting the metadata with blank fields, because there was no text in the input boxes:

add_action( 'save_post', 'album_meta_save' );
function album_meta_save( $id )
{
    // do stuff, including overwrite the custom fields with update_post_meta
}

So I ended up doing the following in a child theme:

function my_undo_hooks( ) {
    remove_action( 'save_post', 'album_meta_save' );
}
add_action( 'after_setup_theme', 'my_undo_hooks' );
add_action( 'save_post', 'my_album_meta_save' );
function my_album_meta_save( $id )
{
    // duplicate the parent theme's function, MINUS the overwriting of the custom fields
}

Now it works just fine. Thanks for the questions and comments - it helped me search out the right answer.