WP_Query with several meta_query-statements and order by meta_value

Question

This is my query:

$the_query = new WP_Query(
    array (
        'post_type'         => 'pass',
        'posts_per_page'    => '-1',
        'post_status'       => 'publish',
        'orderby'           => 'meta_value',
        'meta_key'          => 'start_time',
        'order'             => 'ASC',
        'meta_query'        => array(
            'relation'      => 'OR',
            array(
                'key'       => 'event',
                'value'     => $tmp_posts->ID,
                'compare'   => '=='
            ),
            array(
                'key'       => 'meeting_type',
                'value'     => 'other',
                'compare'   => '!='
            )
        )
    )
);

My problem is this query does not order the posts by start_time. If I remove the second meta_query array (and the 'relation' => 'OR') then the orderby works great. Why is that?

If I var_dump $the_query this is the query outputted:

SELECT wp_posts.*
FROM wp_posts
INNER JOIN wp_postmeta ON wp_posts.ID = wp_postmeta.post_id
INNER JOIN wp_postmeta AS mt1 ON (wp_posts.ID = mt1.post_id)
INNER JOIN wp_postmeta AS mt2 ON (wp_posts.ID = mt2.post_id)
WHERE 1=1 AND wp_posts.post_type = 'pass'
    AND (wp_posts.post_status = 'publish')
    AND (wp_postmeta.meta_key = 'start_time'
        OR (mt1.meta_key = 'event'
        AND CAST(mt1.meta_value AS CHAR) = '10011'
    )
    OR (
        mt2.meta_key = 'meeting_type'
        AND CAST(mt2.meta_value AS CHAR) != 'other')
    )
GROUP BY wp_posts.ID
ORDER BY wp_postmeta.meta_value ASC

How can I get this to order my posts by start_time?

Answer 1

I was unable to create a WP_Query – I believe it is not possible to solve this using a regular WP_Query – but managed to create an SQL statement that does exactly what I want:

SELECT wp_posts.*
FROM wp_posts

INNER JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)
INNER JOIN wp_postmeta AS mt1 ON (wp_posts.ID = mt1.post_id)

WHERE 1=1 
    AND wp_posts.post_type = 'pass'
    AND (wp_posts.post_status = 'publish')
    AND (
        wp_postmeta.meta_key = 'start_time'
        AND (
            (
                mt1.meta_key = 'event'
                AND CAST(mt1.meta_value AS CHAR) = '10011'
            )
            OR (
                mt1.meta_key = 'meeting_type'
                AND CAST(mt1.meta_value AS CHAR) != 'other'
            )
        )
    )

GROUP BY wp_posts.ID
ORDER BY wp_postmeta.meta_value ASC