One recent post from each category

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 http://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.

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)?

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!

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.