WP Query group/order by category name

Question

There are a few ways you can do this.

The simplest is to just get everything: posts and terms, then use some array_filter magic to group things. Simple example (that will only work with PHP 5.3+):

<?php
$terms = get_terms('your_taxonomy');

$term_ids = array_map(function($t) {
    return $t->term_id,
}, $terms);

$posts = get_posts(array(
    'nopaging'      => true,
    'tax_query'     => array(
        array(
            'taxonomy'  => 'category',
            'field'     => 'id',
            'terms'     => $term_ids,
        ),
    ),
    'meta_query'    => array(
        array(
            'key'       => 'TEST1',
            'compare'   => 'EXISTS', // wp 3.5+ only
        )
    ),
));

foreach ($terms as $t) {
    $posts_in_term = array_filter($posts, function($p) use ($t) {
        // has_term likely triggers a DB hit...
        return has_term($t->term_id, 'your_taxonomy', $p);
    });

    // do stuff with $posts_in_term 
}

This is fairly easy to understand. You take a lot of the ordering work you would have sent to the DB and did it in the PHP app itself. Not bad, certainly fewer DB hits than you were doing. If you need to put a headline with each category name, this would likely be the way to go because it’s going to be very difficult with the next method.

Option 2: Do the get_posts query as normal, but hook into posts_groupby and order by term ID. You need need to do some digging to figure out the table aliases/names to order by in the WP_Tax_Query class which creates table aliases based on how many taxonomy queries are in a given set. Since we only have one, there is no allias and you just need to add $wpdb->term_relationships.object_id to the group by. The end result is something that looks like GROUP BY $wpdb->posts.ID, $wpdb->term_relationships.object_id.

Example:

<?php
// our group by callback
function wpse84243_groupby($groupby, $query) {
    global $wpdb;

    return $groupby . ' ' . $wpdb->term_relationships . '.object_id';
}

add_filter('posts_groupby', 'wpse84243_groupby', 10, 2);
$posts = get_posts(array(
    'nopaging'      => true,
    'tax_query'     => array(
        array(
            'taxonomy'  => 'category',
            'field'     => 'id',
            'terms'     => get_terms('category', array('fields' => 'ids')),
        ),
    ),
    'meta_query'    => array(
        array(
            'key'       => 'TEST1',
            'compare'   => 'EXISTS', // wp 3.5+ only
        )
    ),
));

// remove the filter, put things back to normal
remove_filter('posts_groupby', 'wpse84243_groupby', 10, 2);

// do stuff with $posts, they'll be grouped by your taxonomy ID.

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