How to second orderby in "pre_get_posts" by meta value or combine single date and time to timestamp

Question

This is somewhat of a continued problem from another question that I had asked. Here is the original post. pre_get_post filter returns results when there should not be

Since then I added an orderby clause....

  if ($post_ids) {
                $wp_query->set('post__in', $post_ids);
                $wp_query->set('meta_key', '_start_date');
                $wp_query->set('orderby','meta_value');
                $wp_query->set('order', 'ASC');
            }else{
                $wp_query->set('post__in', array(0));

            }
            return $wp_query;

Problem is that I need to order by another meta_key's value. The meta_key is'_start_time'.

The $post_ids are compiled from another (complex)query and then I needed to compile that even more based on other factors. What I ended up with was the post ids that I needed and now I just need to display them in the order that I need. I have the order by _start_date working, but if there are more than one post on that date then they are not ordering by the right time that I need. So I also need to order based off the '_start_time' meta_key value, but can't seem to find a solution for this. Thanks for any help you can provide.

Answer 1

I'm outlining a solution for what you asked in your last comment, because this wouldn't been fitting for the comment format.

Code:

function date_time_to_timestamp_meta( $post_id ) {

    // meta fields are saved in DATETIME, e.g. »2013-12-09 22:32:12«
    // here is only the date part relevant
    $datetime_date = get_post_meta( $post_id, '_start_date', true );
    // here is only the time part relevant
    $datetime_time = get_post_meta( $post_id, '_start_time', true );

    // we're extracting the relevant part with date()
    $extracted_date = date('Y-m-d', $datetime_date);
    $extracted_time = date('G:i:s', $datetime_time);

    // we're bringing back together the extracted relevant date and time
    $datetime_date_and_time = $extracted_date . ' ' . $extracted_time;

    // we're constructing the TIMESTAMP with strtotime()
    $date_time_timestamp = strtotime( $datetime_date_and_time );

    // we're updating/creating our TIMESTAMP meta field
    update_post_meta( $post_id, '_date_time_timestamp', $date_time_timestamp ); 

}   
add_action( 'save_post', 'date_time_to_timestamp_meta', 111 );

Note: exemplary and untested

As this is just a proof of concept above function for sure is missing some things you normally want to have, like conditional checks on post type and/or meta fields. Read into the according php functions, if you're not sure how to use them. I assumed a high priority for this, not knowing how and when the API calls you're talking about are happening, in short, you of course want to make sure this is happening after the two single date and time fields are available with the actual information you need.

Answer 2

I'm not completely sure, but either it's white space separated, or ,/comma separated. One of those two options should work.