此查询几乎按预期工作，但由于某些原因，我无法按自定义字段将其排序opl_submission_tempo. 该字段设置为一个数字，所以我不确定这里哪里出错了。

$tempo_query = new WP_Query(
    array(
      \'post_type\'       => \'opl_tempo_submission\',
      \'order_by\'        => \'meta_value_num\',
      \'meta_key\'        => \'opl_submission_tempo\',
      \'order\'           => \'ASC\',
      \'posts_per_page\'  => 1,
      \'meta_query\'      => array(
        \'relation\' => \'AND\',
        array(
          \'key\' => \'opl_submission_exercise_type\',
          \'value\' => $exercise_type,
          \'compare\' => \'=\'
        ),
        array(
          \'key\' => \'opl_submission_exercise_number\',
          \'value\' => $exercise_number,
          \'compare\' => \'=\'
        )
      )
    )
  );