Validation\Validator\Uniqueness

use Phalcon\Validation\Validator\Uniqueness as UniquenessValidator;

   $validator->add('username', new UniquenessValidator([
       'model' => new Users(),
       'message' => ':field must be unique'
   ]));

Different attribute from the field:

$validator->add('username', new UniquenessValidator([
       'model' => new Users(),
       'attribute' => 'nick'
   ]));

In model:

$validator->add('username', new UniquenessValidator());

Combination of fields in model:

$validator->add(['firstName', 'lastName'], new UniquenessValidator());

It is possible to convert values before validation. This is useful in situations where values need to be converted to do the database lookup:

$validator->add('username', new UniquenessValidator([
     'convert' => function (array $values) {
         $values['username'] = strtolower($values['username']);

         return $values;
     }
 ]));