Phalcon validation 2 fields and concat

Hi,

I have 2 input fields, the first input is a prefix and the second is the number, example : prefix -> AB, number ->005.

I pass this parameter by post and server side I'll concat this data separated by - for example : AB-005.

I want to add to my custom validation class the presence of the prefix field and after the number field. And I want to add the Uniqueness validation on the full string AB-005to check if I have this in my database. Soooo... I did that

$this->add(
    'prefix',
    new PresenceOf(
        array(
            'message' => 'The prefix is required'
        )
    )
);

$this->add(
    'number',
    new PresenceOf(
        array(
            'message' => 'The number is required'
        )
    )
);

$this->add(
    'prefix'.'-'.'number',  // <--------- I don't know how I can do that...
    new Uniqueness(
        array(
            'model' => 'Users',
            'message' => 'This data should be unique'
        )
    )
);

Do you know how I can do to concat the prefix and number data to add the Uniqueness validation ?

Edit : replaced $validation by $this