WP 4.2 will include a number of improvements in the way query classes handle ‘orderby’, particularly when used in connection with the ‘meta_query’ parameter.

WP_Query has long supported the sorting of results by postmeta, using the following syntaxes:

$q1 = new WP_Query( array(
    'meta_key' => 'foo',
    'orderby' => 'meta_value',
) );

$q2 = new WP_Query( array(
    'meta_key' => 'foo',
    'orderby' => 'foo',
) );

These arguments have always operated more or less independently from ‘meta_query’. Internally, ‘meta_key’ and ‘meta_value’ are converted to the first clause in a ‘meta_query’, but this conversion is opaque as far as ‘orderby’ is concerned.

WP 4.2 will address this shortcoming with a new syntax for ordering by specific clauses of a ‘meta_query’ array. When building the ‘meta_query’ parameter, use explicit array indexes for the meta query clauses. These indexes will be used as handles, which can be referenced from the ‘orderby’ param. For example:

$q = new WP_Query( array(
    'meta_query' => array(
        'relation' => 'AND',
        'state_clause' => array(
            'key' => 'state',
            'value' => 'Wisconsin',
        ),
        'city_clause' => array(
            'key' => 'city',
            'compare' => 'EXISTS',
        ), 
    ),
    'orderby' => 'city_clause', // Results will be ordered by 'city' meta values.
) );

This new syntax can be used in conjunction with complex ‘orderby’ parameters:

// ...
    'orderby' => array(
        'city_clause' => 'ASC',
        'state_clause' => 'DESC',
    ),
// ...

See #31045 for more background.

We’ve also done a sweep through WP’s primary query classes – WP_Query, WP_User_Query, and WP_Comment_Query – to ensure that ‘orderby’ can be used in the same way everywhere. As of 4.2, ‘orderby’ syntax – including multidimensional arrays, ‘meta_value_num’, and the meta query index references described above – is the same in each of these three query classes. See #31265.

#4-2, #dev-notes, #post-meta, #query