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I'm trying to create a list of parent IDs for a custom post type for use within a "must use" plugin I'm developing on a project.

Here's the code I'm using to generate the query:

$cpt_parents = new WP_Query( array ( 
    'post_type' => 'cpt',
    'posts_per_page' => -1, // Get them all 
    'post_parent' => 0, // Return only top-level        
    'fields' => 'ids'

$parent_ids = $cpt_parents->posts;

And here are the resulting errors:

Notice: Undefined index: wp_taxonomies in [url]/wp-includes/query.php on line 1700

Warning: Invalid argument supplied for foreach() in [url]/wp-includes/query.php on line 1700

Notice: Undefined index: wp_taxonomies in [url]/wp-includes/query.php on line 1700

Warning: Invalid argument supplied for foreach() in [url]/wp-includes/query.php on line 1700

Fatal error: Call to undefined function is_user_logged_in() in [url]/wp-includes/query.php on line 2485

What do I need to do in order to run WP_Query within a plugin?

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I think it may have something to do with this issue: core.trac.wordpress.org/ticket/20904 – Jonathan Wold Nov 12 '12 at 20:37
Is that code alone in the plugin? Or inside a hook callback? If so, please provide the context... – brasofilo Nov 12 '12 at 20:38
That code is alone in the plugin. I've also tried it within a function tied to the "init" action. It works great on the front-end, but throws the same errors on the backend. I'm pretty sure it's because the mu-plugin is being run prior to init, which appears to be a requisite, so far, for WP_Query. – Jonathan Wold Nov 12 '12 at 20:43
I'm really not getting it. Why would you run some unhooked code like this? Where exactly do you need it to run? Does this references (1 - 2) shred some light? – brasofilo Nov 12 '12 at 22:03
I'm using WPAlchemy to create an interface that appears only on the "parent" of a custom post type. WPAlchemy has an include function that accepts an array of IDs. I'm trying to use the query above to generate that list of IDs. The problem is that all of this is run before "init" is called, which is preventing WP_Query from running properly. – Jonathan Wold Nov 12 '12 at 22:53
up vote 1 down vote accepted

I figured it out. I moved the reference to WP Alchemy within a function that fires after init. Everything now works great and I can use WP_Query within the setup files.

Here's a code reference:

// WP Alchemy Setup
function jd_setup_wp_alchemy() {
    include_once WPMU_PLUGIN_DIR . '/jonathanwold/metaboxes/setup.php';
    include_once WPMU_PLUGIN_DIR . '/jonathanwold/metaboxes/cpt-spec.php';
add_action('init', 'jd_setup_wp_alchemy');
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