Automatically adding meta data to posts or multiple query help

This will query all the ticket posts and update a _ticket_date field. However, I’m not yet sure how to retrieve the actual date, so you’ll have to provide me with a little more information for that. Assuming it works properly I would run the following once, and then remove. You could achieve a similar effect by using setting a transient but I won’t get into that now.

function update_ticket_meta(){

  $args = array( 'post_type' => 'ticket', 'nopagination' => true );
  $tickets = new WP_Query( $args );

  if ( $tickets->have_posts() ) while( $tickets->have_posts() ){
    $tickets->the_post();

    // get the event ID- adjust meta key as needed
    $event_id = get_post_meta( $post->ID, '_event', true );

    // get the date from the event post
    $event = Ai1ec_Events_Helper::get_event($event_id);

    if( $event ) {
       // get date in desired format- YYYYMMDD should be the easiest to query by
       $date = date( 'ymd', $event->start );

       if( $date )
        update_post_meta( $post->ID, '_ticket_date', $date );
    }

  }
}
add_action( 'admin_init', 'update_ticket_meta' );

Additionally, to save the meta whenever a ticket is saved you can add an action to save_post-{post-type}. This will still leave you some coding to do if you change the event’s date meta field.

// save some meta when a ticket is saved
function add_ticket_date( $post_id ){

    if( !class_exists( 'Ai1ec_Events_Helper' ) )
          return;

    // get the event ID- adjust meta key as needed
    $event_id = isset( $_POST['_event'] ) ? (int) $_POST['_event'] : -1;

    // get the date from the event post- again adjust meta key
    $event = Ai1ec_Events_Helper::get_event($post->ID);

    // get date in desired format- YYYYMMDD should be the easiest to query by
    $date = date( 'ymd', $event->start );

    if( $date )
        update_post_meta( $post_id, '_ticket_date', $date );


}
add_action( 'save_post-ticket', 'add_ticket_date' );