Wordpress - Nested meta_query with multiple relation keys

The question was for Wordpress 3.0, but just in case someone has the same question for a more recent version, from Wordpress Codex:

"Starting with version 4.1, meta_query clauses can be nested in order to construct complex queries."

https://developer.wordpress.org/reference/classes/wp_query/#custom-field-post-meta-parameters So, that query should work on the current Wordpress version.

That seems to be impossible. Please someone correct me if I'm wrong.

The meta_query parameter will actually be transformed into a WP_Meta_Query object, and the relation verification won't go deeper in wp-includes/meta.php, and occurs just once in the top level:

if ( isset( $meta_query['relation'] ) && strtoupper( $meta_query['relation'] ) == 'OR' ) {
    $this->relation = 'OR';
} else {
    $this->relation = 'AND';
}

A possible solution for this is to build your own JOIN for this query.

$query = new WP_Query( array(
    ...
    'my_meta_query' => true,
    'suppress_filters' => false
) );

add_filter( 'posts_join', 'my_meta_query_posts_join', 10, 2 );
function my_meta_query_posts_join( $join, $query ) {

    if ( empty( $query->query_vars['my_meta_query'] ) )
        return $join;

    global $wpdb;

    $new_join = "
        INNER JOIN {$wpdb->postmeta} pm1 ON 1=1
            AND pm1.post_id = {$wpdb->posts}.ID
            AND pm1.meta_key = '_some_meta_key'
            AND pm1.meta_value = 'some_value'
    ";

    return $join . ' ' . $new_join;
}

And if you need further verifications and rules, you can also use the posts_where filter.

Meanwhile this is possible, see documentation with example and explanation:

https://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters

and another example https://wordpress.org/support/topic/wp_query-with-multiple-meta_query/#post-9410992

'meta_query'  => array(
    'relation' => 'OR',
    array(
        'relation' => 'AND',
        array(
            'key'     => '_price',
            'value'   => 1,
            'compare' => '>=',
            'type' => 'DECIMAL',
        ),
        array(
            'key'     => '_price',
            'value' => 3000,
            'compare' => '<=',
            'type' => 'DECIMAL',
        ),
    ),
    array(
        'relation' => 'AND',
        array(
            'key'     => '_price',
            'value'   => 3001,
            'compare' => '>=',
            'type' => 'DECIMAL',
        ),
        array(
            'key'     => '_price',
            'value' => 6000, //fixed <= to =>
            'compare' => '<=',
            'type' => 'DECIMAL',
        ),
    )
),