Zend Framework: How to subtract amount from database table field?

I want to subtract a amount from a t开发者_JAVA百科able's field in Zend Framework. I can run it with SQL using following query:

UPDATE `Person` SET credit=credit-50 where id=1

But how to write above SQL query in Zend Framework?

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$row = array('credit' => new Zend_Db_Expr('credit - 50'));
$where = "id = 1";
$nRowsAffected = $db->update('Person', $row, $where);

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Use Zend_Db_Expr

  $data = array(
      'credit'      => new Zend_Db_Expr('credit - 50'),
  );
  $n = $db->update('Person', $data, 'id=1');