Using tax_query to get single post per category

Question

I have a list of magazines I need to categorize and sort. I have created a custom post type and registered a custom taxonomy to organize them.

Category Structure: Print Magazines (parent category)

• Animals (child category)
  • Dog (magazine)
  • Cat (magazine)
  • Bird (magazine)
• Sports (child category)
  • Baseball (magazine)
  • Football (magazine)
  • Soccer (magazine)

I need to query the print-magazine category and create a list of print magazine categories [animals, sports, tech, ...]. Each child category will use the first magazines cover image as the child categories thumbnail. That thumbnail will link to a page that lists all magazines for that category. That page will link to individual magazine pages.

I can query and get all magazines in the parent category (print-magazines), but am wondering how to return only the first magazine post info per child category or loop through the results of the query to sort it myself how I see fit.

This is my tax query.

$args = array(
    'post_type' => 'magazine',
    'post_status' => 'publish',
    'posts_per_page' => -1,
    'tax_query' => array(
        array(
            'taxonomy' => 'magazine-category',
            'field' => 'slug',
            'terms' => 'print-magazines'
        )
    )
);

$the_query = new WP_Query( $args );

while ( $the_query->have_posts() ) : $the_query->the_post();
    $image = get_field('magazine_thumbnail');
    if( !empty($image) ) {
        // render result to page
    }
endwhile;

Answer 1

Set 'posts_per_page' to 1 on your args variable. I also suggest that you add 'page-attributes' while registering the magazine's post type and order the result by 'menu_order'. This way, you will have the field Order while editing a magazine, and set the highest which should appear on this query.