Adding custom validation in zend framework
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I have a mysql table named users having fields: username, password, email
I have controller/action like this 'user/update' for adding new user and 'user/update/id/{user_id}'
I have same Zend_Form for both controller. For adding new user, I've checked whether the username already exist or not using below code:

    $username->addValidator('Db_NoRecordExists', true, array('table' => 'user', 'field' => 'username'));

This works well when new user is added but when editing the user, even when the user leaves the username field as it is, i get username already exist error. 
Is their a validation that zend provide similar to Db_NoRecordExists for editing the field.
In editing case,i want query like:

    SELECT COUNT(*) FROM users WHERE username='$username' AND id!=$update_id

How can i do this?