Find Posts based on Child Post value

Question

I am creating a plugin that adds two custom post types, one of which will always be a child of the other. The parent post type is for Events, the child for Performances. Before entering the Loop, I'd like to be able to find a set of Events that have Performances on a specific date. Performance date is stored as a meta field on the Performance post. The Loop would then list each Event matched, like in a standard post archive loop.

My preference is to query the Event post type, since that strikes me as easier to manage for pagination, but if there's a better implementation that simply involves querying performances, I am open to that, but I'd like to see if there's a facility for limiting a query based on a field in the child post.

Answer 1

Child posts are related to their parents (through the "post_parent" field), and not vice versa. So you would have to query your child post type first, then get the parents of all the results.

// obviously these variable names and key names might not apply,
// change them as necessary
$performances = get_posts( array(
     'meta_key' => 'the_performance_date_custom_field',
     'meta_value' => $date,
     'post_type' => 'performance' ) );

// extract the parent post ID's from each of the returned performances   
$event_ids = array_map( 
     create_function('$post','return $post->post_parent'), 
     $performances );
$event_ids = array_unique( $event_ids ); // dump any duplicate events

// and finally, get the events from their ids
$events = get_posts( array(
    'post_type' => 'event',
    'post__in' => $event_ids ) );