how do i get only 1 post from each category in wordpress

Question

i have a category named news and many subcategories inside it. What i wanna do is to get only 1 posts(newest) from each of those sub categories(including category title, pos

Answer 1

<?php

$news_cat_ID = get_cat_ID( 'News' ); 
$news_cats   = get_categories( "parent=$news_cat_ID" );
$news_query  = new WP_Query;

foreach ( $news_cats as $news_cat ) :
    $news_query->query( array(
        'cat'                 => $news_cat->term_id,
        'posts_per_page'      => 1,
        'no_found_rows'       => true,
        'ignore_sticky_posts' => true,
    ));

    ?>

    <h2><?php echo esc_html( $news_cat->name ) ?></h2>

    <?php while ( $news_query->have_posts() ) : $news_query->the_post() ?>

            <div class="post">
                <?php the_title() ?>
                <!-- do whatever you else you want that you can do in a normal loop -->
            </div>  

    <?php endwhile ?>

<?php endforeach ?>

Answer 2

After a few hours and thanks to our fellows all around the globe, I've been able to modify the main query so we don't even need to go the templates and generate new queries and loops...

// My function to modify the main query object
function grouped_by_taxonomy_main_query( $query ) {

    if ( $query->is_home() && $query->is_main_query() ) { // Run only on the homepage

        $post_ids = array();

        $terms = get_terms('formato');

        foreach ( $terms as $term ) {
            $post_ids = array_merge( $post_ids, get_posts( array( 
                'posts_per_page' => 4, // as you wish...
                'post_type' => 'video', // If needed... Default is posts
                'fields' => 'ids', // we only want the ids to use later in 'post__in'
                'tax_query' => array( array( 'taxonomy' => $term->taxonomy, 'field' => 'term_id', 'terms' => $term->term_id, )))) // getting posts in the current term
            );
        }

        $query->query_vars['post_type'] = 'video'; // Again, if needed... Default is posts
        $query->query_vars['posts_per_page'] = 16; // If needed...
        $query->query_vars['post__in'] = $post_ids; // Filtering with the post ids we've obtained above
        $query->query_vars['orderby'] = 'post__in'; // Here we keep the order we generated in the terms loop
        $query->query_vars['ignore_sticky_posts'] = 1; // If you dont want your sticky posts to change the order

    }
}

// Hook my above function to the pre_get_posts action
add_action( 'pre_get_posts', 'grouped_by_taxonomy_main_query' );