WordPress pre_get_posts not working

Question

Some background:

• I'm trying to replace the default main query for archive templates, and to return a subset of a custom post type ('days') where meta values against the key 'dates' is between a certain value
• The 'days' custom post type has a custom variable field of 'dates' which stores individual database rows in the standard wp_postmeta table for each date associated with that post, stored in Epoch format ('U')
• I'm fairly confident that the SQL query is functioning correctly; the post IDs it returns appear to be correct in each case for the date range queried.

I'm running the following query in my functions file:

   function modify_queries( $query ) { 
        global $wpdb;
        if ( $query->is_post_type_archive('days') && $query->is_main_query() ) {
            // Get Days with a date within the archive range
            $year = substr($query->query_vars['m'],0,4);
            $month = substr($query->query_vars['m'],4,2);
            $day = substr($query->query_vars['m'],6,2);

            if (is_year()) {
                $startDate = date('U', mktime(0, 0, 0, 1, 1, $year));
                $endDate = date('U', mktime(0, 0, 0, 12, 31, $year));
            }
            if (is_month()) {
                $startDate = date('U', mktime(0, 0, 0, $month, 1, $year));
                $endDate = date('U', mktime(0, 0, 0, $month, cal_days_in_month(CAL_GREGORIAN, $month, $year), $year));
            }
            if (is_day()) {
                $startDate = date('U', mktime(0, 0, 0, $month, $day, $year));
                $endDate = date('U', mktime(0, 0, 0, $month, $day, $year));
            }

            $request  = "SELECT ID FROM $wpdb->posts, $wpdb->postmeta";
            $request .= " WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id";
            $request .= " AND post_status='publish' AND post_type='days'";
            $request .= " AND $wpdb->postmeta.meta_key = 'dates' AND $wpdb->postmeta.meta_value >= $startDate AND $wpdb->postmeta.meta_value <= $endDate LIMIT 0 , 30";
            $postsList = $wpdb->get_results($request,ARRAY_N);
            if ($postsList) {


                foreach ($postsList as $thePost) {
                    $thePostList[] = $thePost[0];
                }
                $query->set( 'post__in', $thePostList);
                $query->set( 'post_type', 'days');

                return;
            } else {
                return false;
            }

}
add_action( 'pre_get_posts', 'modify_queries',1 );

My understanding is that this should take the output of the query and simply replace the normal query with one using the IDs returned from the SQL query - however, it's just returning the 5 most recent 'days' posts.

At present, the archive.php file consists purely of the following, so we can rule out any interference:

<?
while ( have_posts() ) : the_post();
global $post;
the_title();
endwhile;
?>  

I get the 5 most recently published 'days' posts, and it seems to ignore the IDs that the pre_get_posts filter should have passed it.

I have a sneaky suspicion that part of the problem relates to setting 'post_id' on the main query, rather than 'p', but I don't seem to get any results at all with the latter.

I've hunted around, but can't find much documentation about pre_get_posts other than how useful and beneficial it is as a replacement to query_posts.

Live debug output at: http://dev.daysoftheyear.com/days/2012/10

Answer 1

You can find the documentation on the pre_get_post filter here http://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts

It looks to me like the problem is in fact how you are setting the post_ids that you want to retrive. If you refer to the link above, you can see the different values that can be set on the $query object that is passed to the filter, and post_id isn't one of them, which is why it "works" when you set it, and "doesn't work" when you set p. The latter is for a single post ID, so if you just wanted 100, you would use $query->set('p', 100). If you want to return results where the post ID is in an array, you use $query->set('post__in', array(100, 120)). Your SQL code is just returning the ID, so rather than returning objects, you can just get an array directly - I'm assuming that echoing the title and print_r lines are just debugging:

// get results as a numeric array
$post_ids = $wpdb->get_results($request, ARRAY_N);
// pass post id array to $query
$query->set( 'post__in', $post_ids );

You may also not get results if there are existing conflicting parameters set on the $query. You can check these values with var_dump($query->query_vars) and set any that may be in conflict to an empty string - in your case $query->set( 'm', '' );

Answer 2

Think I've nailed it; gave 'm' a blank value to stop it thinking it was a date-based query. Thanks! :)