Grails domain class: unique constraint for multiple columns

Suppose a simple Grails domain class:

class Account {
    String countryId;

    String userName; 

    String areaId;

    String password;

    static constraints = {
        ...???...
    }
}

It is required that user names are unique for a particular countryId and areaId, thus there must be a unique contraint on three columns. And this is the constraints:

userName(unique: ['countryId', 'areaId'])

You can include as many other properties in the array that make up the 
other properties that must be considered in the "unique" constraint on 
the username.