我认为这样的事情应该可以做到：

$meta_query = array(
    array(
         \'key\'     => \'foo\',
         \'compare\' => \'EXISTS\',
    ),
);

$args = array(
    \'meta_query\' => $meta_query
);

$query = new WP_Query( $args );

if ( $query->have_posts() ) {
    while ( $query->have_posts() ) {
        $query->the_post();
        $post_id = get_the_ID();
    }
} else {
    // no posts found
}

wp_reset_postdata();