One recent post from each category

why not just use wordpress’s built in functions? you could grab all categories, and then foreach category, grab the most recent post. let us know if you need help doing this.

Yes, initially did.

But I’m stopped on pagination and also saw the problem that foreach category will a lot of sql queries.

In general I thought it would be convenient to use a WP loop, because due to this will be available built-in WP features.

Now I’ve created the class (based on the https://wordpress.stackexchange.com/questions/3708/custom-taxonomy-wp-query-for-all-terms-in-a-taxonomy example):

class Recent_in_Taxonomies extends WP_Query {
	var $taxonomy;
	function __construct ( $args = array() ) {
		add_filter ( 'posts_join', array( &$this, 'posts_join' ), 10, 2 );
		$this->taxonomy = $args['taxonomy'];
		parent::query ( $args );
	}
	function posts_join( $join, $query ) {
		global $wpdb;
		$join .=<<<SQL
JOIN( SELECT MAX( {$wpdb->posts}.post_date ) AS post_date FROM {$wpdb->posts}
LEFT JOIN {$wpdb->term_relationships} AS rel ON {$wpdb->posts}.ID=rel.object_ID
LEFT JOIN {$wpdb->term_taxonomy} AS tax USING( term_taxonomy_id )
LEFT JOIN {$wpdb->terms} AS trm USING( term_id )
WHERE tax.taxonomy='{$this->taxonomy}' GROUP BY trm.term_id ) AS rmt USING( post_date )
SQL;
		return $join;
	}
}

and call it:

$args[ 'post_type' ] = 'post';
$args[ 'taxonomy' ] = 'category';
$args[ 'paged' ] = $paged;

$loop = new Recent_in_Taxonomies( $args );

get_template_part ( 'loop' );

while ( have_posts() ) : the_post();

Everything seems to be ok.

But now is a new huge problem: query recognizes that the post can be simultaneously in multiple categories, and loop receive post__in = 1, 2, 45, 1, 45, 1.

Yes, I need to get in the loop pages including duplicates (I did this query for). But loop only shows unique post, and I’m not yet figured out what to do.

so if a post is the most recent in two categories, it only shows that post for the first category, and then the second category it shows the second most recent post?

OMG, I was stupid.

But now understand:

if a post is the most recent in two categories, it only shows that post for the first category

– found that there were just the general recent posts list, but it was the result of default wp_query on home page! My query was out after and I have not seen it result.

I rewrote the code, my index.php now is:

<?php

get_header();

if  ( is_home() ) : // Get latest posts from each category

	function recent_join ( $join, $query ) {
		global $wpdb;
		$taxonomy = 'category';
		$join .=<<<SQL
JOIN( SELECT MAX( {$wpdb->posts}.post_date ) AS post_date, trm.term_id AS term_ID FROM {$wpdb->posts}
LEFT JOIN {$wpdb->term_relationships} AS rel ON {$wpdb->posts}.ID=rel.object_ID
LEFT JOIN {$wpdb->term_taxonomy} AS tax USING( term_taxonomy_id )
LEFT JOIN {$wpdb->terms} AS trm USING( term_id )
WHERE tax.taxonomy='{$taxonomy}' GROUP BY trm.term_id ) AS pmx USING( post_date )
SQL;
		return $join;
	}
	add_filter ( 'posts_join', 'recent_join', 10, 2 );

	function recent_fields ( $fields, $query ) {
		global $wpdb;
		return $fields . ', term_ID';
	}
	add_filter ( 'posts_fields', 'recent_fields', 10, 2 );

	$args = array (
		'post_type' => 'post',
		'taxonomy' => 'category',
		'paged' => get_query_var ( 'paged' ) ? get_query_var ( 'paged' ) : 1
	);
	$wp_query = new WP_Query ( $args );

endif; // ------------------------------------------------

get_template_part ( 'loop' );

get_footer();

?>

and it working!

But unfortunately the default wp_query is still running in front of my cycle (although it isn’t needed, not used).
Please tell me whether and how to replace with my own (or disable) default wp_query (on homepage)?

you would need to add a function to run on a hook that takes place before the query, and detect the user is viewing the home page, and disable the default query that way.

already unsuccessfully tried ( w/o hook ) at the functions.php

what the hook’s name ( init ? ) and where is the ‘place before’ default wp_query? 10x!

here is a list of hooks, in chronological order: https://codex.www.ads-software.com/Plugin_API/Action_Reference

Great, I placed it in a ‘parse_query’ hook.

Now looking for another way to run query only on home page, because is_home == TRUE only after performing wp_query.

Found solution:

function recent_get_posts ( &$wp_query ) {
		if ( is_home() ) :
			static $first_call = true;
			if ( ! $first_call ) return;
			else $first_call = false;
			$wp_query->query_vars = array(
				'post_type' => 'post',
				'taxonomy' => 'category',
				'paged' => get_query_var( 'paged' ) ? get_query_var( 'paged' ) : 1
			);
			function recent_join( $join, $query ) {
				global $wpdb;
				$taxonomy = 'category';
				$join .=<<<SQL
JOIN( SELECT MAX( {$wpdb->posts}.post_date ) AS post_date, trm.term_id AS term_ID FROM {$wpdb->posts}
LEFT JOIN {$wpdb->term_relationships} AS rel ON {$wpdb->posts}.ID=rel.object_ID
LEFT JOIN {$wpdb->term_taxonomy} AS tax USING( term_taxonomy_id )
LEFT JOIN {$wpdb->terms} AS trm USING( term_id )
WHERE tax.taxonomy='{$taxonomy}' GROUP BY trm.term_id ) AS pmx USING( post_date )
SQL;
				return $join;
			}
			add_filter( 'posts_join', 'recent_join', 10, 2 );
			function recent_fields( $fields, $query ) {
				global $wpdb;
				return $fields . ', term_ID';
			}
			add_filter( 'posts_fields', 'recent_fields', 10, 2 );
		endif;
	}
	add_action ( 'pre_get_posts', 'recent_get_posts' );

glad it works ?? *mark as resolved*