Get terms ordered by post date

Question

What I'm trying to achieve: get n terms from a custom taxonomy ordered by the newest post in each term. This will be used to display the newest m posts under each term.

Is there a better way of doing this than getting an array of all the terms, and looping through to find the newest post in each? This approach doesn't feel very efficient or scalable.

Update

Here's my working code so far - can this be improved on?

$taxonomy = 'industry'; //taxonomy name
$terms_to_display = 5; //number of terms to display
$posts_per_term = 4; // number of posts per term to display
$terms = get_terms( array('taxonomy' => $taxonomy, 'hide_empty' => true, 'fields' => 'ids' ) );
$news_items = [];

foreach ( $terms as $term ) {

    $args = array (
        'post_type'         => 'news',
        'posts_per_page'    => $posts_per_term,
        'orderby'           => 'date',
        'order'             => 'DESC',
        'tax_query' => array (
            array (
                'taxonomy' => $taxonomy,
                'field'    => 'id',
                'terms'    => array($term),
            ),
        ),
    );

    $news = new WP_Query( $args );

    if ( $news->post_count == $posts_per_term ) { // ignore terms with not enough posts
        $news_items[$term] = get_the_date("U", $news->posts[0]->ID); //get date of newest post in this term
    }

    wp_reset_query();

}

arsort ( $news_items ); //sort descending, keeping keys

$term_ids = array_keys ( array_slice($news_items, 0, $terms_to_display, true) ); //take 'n' newest and return an array of keys (term ids)

foreach ( $term_ids as $term_id ) {

    $term = get_term ( $term_id, $taxonomy );

    echo "<h2>" . $term->name . "</h2>";

    $args = array (
        'post_type'         => 'news',
        'posts_per_page'    => $posts_per_term,
        'orderby'           => 'date',
        'order'             => 'DESC',
        'tax_query'         => array (
                array (
                    'taxonomy' => $taxonomy,
                    'field'    => 'id',
                    'terms'    => array($term_id),
                ),
        ),
    );

    $news = new WP_Query( $args );

    if ( $news->have_posts() ) {    

        echo "<ul>";

        while ( $news->have_posts() ) {
            $news->the_post();

                echo "<li>" . get_the_title() . "</li>";

        }

        echo "</ul>";
    }

    wp_reset_query();

}

Answer 1

Basically, what you are doing right now is this:

1. Get all the terms from industry taxonomy
2. For each term do a WP_Query to get the newest posts, iterate through them to sort the terms, creating a new array with term ids
3. Sort and slice the new array
4. For each element in new array of terms ids do again a WP_Query to get the newest posts, and iterate through them

That's a lot of WP_Queryies. Let's try to lower that number.

We can do it like this:

1. Get all the terms from industry taxonomy
2. For each term do a WP_Query to get the newest posts, iterate through them to sort the terms, creating a new array
3. New array will be keyed with a date in U format, and contain both the term object and the array with post objects, so we won't have to query them again.

Here is my suggested code:

$taxonomy = 'industry'; //taxonomy name
$terms_to_display = 5; //number of terms to display
$posts_per_term = 4; // number of posts per term to display
$terms = get_terms( array('taxonomy' => $taxonomy, 'hide_empty' => true ) );
$news_items = [];

foreach ( $terms as $term ) {

    $args = array (
        'post_type'         => 'news',
        'posts_per_page'    => $posts_per_term,
        'orderby'           => 'date',
        'order'             => 'DESC',
        'tax_query' => array (
            array (
                'taxonomy' => $taxonomy,
                'field'    => 'id',
                'terms'    => array( $term->term_id ),
            ),
        ),
    );

    $news_query = new WP_Query( $args );

    // ignore terms with not enough posts
    if ( $news_query->post_count < $posts_per_term ) {
        continue;
    }

    $news_posts = $news_query->get_posts();

    // get date of newest post in this term
    $newest_post_date = get_the_date( "U", $news_posts[0]->ID );

    $news_items[$newest_post_date] = array(
        'term' => $term,
        'news' => $news_posts,
    );
}

krsort( $news_items ); // sort descending by keys

$news_items = array_slice( $news_items, 0, $terms_to_display );

wp_reset_query();

I have made a few more changes as well:
1. $terms is now array of the WP_Term objects, not just ids
2. Made the $news_query->post_count < $posts_per_term check slightly more readable

And how to use:

foreach ( $news_items as $item ) {

    echo "<h2>" . $item['term']->name . "</h2>";

    echo "<ul>";

    foreach ( $item['news'] as $news_post ) {
        setup_postdata( $news_post );

        echo "<li>" . get_the_title() . "</li>";
    }

    echo "</ul>";
}

wp_reset_query();

This way, we have lowered the number of WP_Queryies to the number of non-empty terms in the industry taxonomy, and made the resulting array much more useful.