How would I use pre_get_posts to query another site for posts in my multisite network?

2 comments

1. OK so for reference I ended up actually using switch_to_blog() and restore_current_blog(). I found no references whatever for querying multiple tables (except writing my own JOIN-statements) when followed the source code for the action pre_get_posts.

   A lot of advice was given to skip restore_current_blog() to save CPU-cycles but when I checked the source code for this I saw that it would cause a huge memory problem since the main thing restore_current_blog() does is to pop the latest inserted object in a global array and this array would grow hugely with each switch_to_blog() that one does not do a restore_current_blog() on.

   Please feel free to check the source code and also see @user42826’s answer regarding this at https://wordpress.stackexchange.com/a/123516/14820.

   For those who are interested, I ended up with transients both for the site-list and for the result to save both CPU-cycles and DB performance. I only needed data to be updated if it were stale (15 minutes or more of age). Only relevant bit of function included below:

   $site_transient = get_site_transient( 'multisite_site_list');
   if ($site_transient == false || LL_USE_TRANSIENTS == false) {
       $using_site_transient = false;
   
       // no cache available
       global $wpdb;
   
       // TODO: get prefix for wp tables
       $site_list = $wpdb->get_results($wpdb->prepare('SELECT * FROM nw_blogs WHERE public = %d AND deleted = %d ORDER BY blog_id', 1, 0));
   
       // set transient
       set_site_transient('multisite_site_list', $site_list, LL_TRANSIENT_EXPIRES);
   
   } else {
       //log_msg("SITELIST - Transient found, using cached query..");
       $site_list = $site_transient;
       $using_site_transient = true;
   }
   
   //log_msg("$id - get_posts_from_network | tags: " . implode(",", $tags) . " | tag_slug__and: $lang_full");
   
   $start = microtime(true);
   
   // get blogpost transient, if existing
   $post_transient = get_site_transient( 'limelight_recent_posts_' . $hash);
   
   // if not, do query and save result as a transient
   if ($post_transient == false || LL_USE_TRANSIENTS == false) {
       $using_post_transient = false;
       $blogposts = array();
   
       foreach ($site_list as $site) {
           switch_to_blog($site->blog_id);
           $posts = get_posts(array(
               'post_status' => 'publish',
               'orderby' => 'date',
               'order' => 'desc',
               'tag' => implode(",", $tags),
               'tag_slug__and' => array($lang_full)
           ));
   
           //log_msg("$id - get_posts_form_network | at site: $site->blog_id");
   
           foreach ($posts as $post) {
   
               //log_msg("$id - get_posts_form_network | at site: $site->blog_id | got post!");
   
               $post->human_post_date = human_time_diff(get_post_time('U', false, $post->ID), current_time('timestamp')) . ' ' . __('ago', 'limelight');
   
               if (isset($tagline_metabox)) {
                   $meta = get_post_meta($post->ID, $tagline_metabox->get_the_id(), TRUE);
                   $post->tagline = $meta['tagline_text'];
               }
   
               if (!isset($post->tagline) || $post->tagline == '')
                   $post->tagline = substr($post->post_title, 0, 20) . '..';
   
               $post->post_date_timestamp = strtotime($post->post_date);           
   
               //$post->blog_id = $site->blog_id;
               $post->blog_name = get_bloginfo('name');
               $post->blog_url = get_bloginfo('wpurl');
               $post->permalink = get_permalink($post->ID);
               $post->author_name = get_userdata($post->post_author)->display_name;
               //$data = get_userdata($post->post_author);
   
               $blogposts[] = $post;
           }
           restore_current_blog();
       }
   
       // sort blogposts by post_date_timestamp, descending
       array_sort_by_obj($blogposts, 'post_date_timestamp', SORT_DESC);
   
       // pick five latest posts
       if (sizeof($blogposts) > 5) {
           $posts = array();
           for ($i = 0; $i <= 4; $i++) {
               $posts[$i] = $blogposts[$i];
           }
           $blogposts = $posts;
       }
   
       // set transient
       if (LL_USE_TRANSIENTS)
           set_site_transient('limelight_recent_posts_' . $hash, $blogposts, LL_TRANSIENT_EXPIRES);
   

2. Try the following:

   add_filter( 'pre_get_posts', 'get_post_from_main_blog' );
   function get_post_from_main_blog($query) {
       global $wpdb;
       $wpdb->posts = 'wp_posts';
       return $query;
   }
   

   You may need to change wp_posts to your main blog’s post table name.

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