Sort wp_query of two post types using meta datefield for one and date for the other - possible?

Question

I have two queries that result in id-arrays that I merge into a common array which, in turn, has all the posts I need from the two post types (posts, events). However, in the sorted list I would like to not use the event's publication "date" for sorting, but the metadata field "event-date". However, posts don't have that field so they should be sorted using the standard "date". Basically, the result should be a list that is sorted by a post-object's meta field event-date when present for the post and the standard publication date if not.

I remember having done something similar a long time ago using SQL directly by creating a common key for sorting. This similar older question also resorts to SQL in the end - Mix post date with post meta value using WP_Query

But remembereing "fallback" arguments in the wp-query arguments, I was wondering if this can also be achieved using wp_query directly. Alas, so far my attempts have been unlucky. The code below is what I've come up with googling for ideas - the meta query is in there so posts without the meta-key are also included.

These query args do return "some order", but a weird one. It's certainly not what I'm looking for. For the few last years I did not have to resort to directly querying SQL in WP, because wp_query had become sufficiently powerful, maybe this problem can also be addressed using the class, even if not by me?

Thanks for every tip!

$args = array(
            'post__in'  => $all_ids,
            'meta_query' => array(
                'relation' => 'or',
                    array(
                        'key'     => '_event_date',
                        'compare' => 'EXISTS',
                    ),
                    array(
                        'key'     => '_event_date',
                        'compare' => 'NOT EXISTS',
                    )
                ),
            'orderby'   => array( 'meta_value_num' => 'DESC', 'date' => 'DESC'),
            'order'     => 'DESC',
        );

Answer 1

Try this WP_Query arguments:

$args = array(
    'post__in'  => $all_ids,
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key'     => '_event_date',
            'compare' => 'EXISTS',
        ),
        array(
            'key'     => '_event_date',
            'compare' => 'NOT EXISTS',
            'value'   => 'dummy_value'  // This is just to ensure this condition gets evaluated.
        )
    ),
    'orderby'   => array( 'meta_value_num' => 'DESC', 'date' => 'DESC' ),
    'order'     => 'DESC',
);