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The data for the field must be unique, it cannot be used by any other rows.

var $validate = array(
    'login' => array(
        'rule' => 'isUnique',
        'message' => 'This username has already been taken.'
    )
);

1. var $validate = array(
2. 'login' => array(
3. 'rule' => 'isUnique',
4. 'message' => 'This username has already been taken.'
5. )
6. );