如何使用Drupal`db_select（）`方法转换查询？

How can I convert this query using db_select() method?

select id, name, (select count(*) from da_pages a where b.id = a.page_id) count, active from da_pages b where page_id=$arg

Tried using addExpression and countQuery() etc.,

$query = db_select('da_pages', 'dp')
      ->fields('dp',array('id','name','active'))
      ->condition('page_id', $arg,'=')
      ->countQuery()
      ->extend('PagerDefault')
      ->limit(5); // line1
      //$query->addExpression('count(select * from da_pages a where db.id = a.page_id)', 'count');
  $result = $query->execute();

Found a solution from the drupal community. Not sure about if this is the exact method, Anyway I got the expected output using the following code

$count = "(select count(*) from da_pages a where dp.id = a.page_id)";
  $query = db_select('da_pages', 'dp')
      ->fields('dp',array('id','name','active'))
      ->condition('page_id', $arg,'=')
      ->extend('PagerDefault')
      ->limit(5); // line1
  $query->addExpression($count, 'count');
  $result = $query->execute();