Validating the uniqueness of the combination of two fields

Is there a built-in method to validate that two fields in combination
are unique?

e.g. if my table is :

username | password

user1 | pass1

the following happens>“user1”,password=>“pass2”) => ok>“user2”,password=>“pass1”) => ok>“user1”,password=>“pass1”) => not ok


I am interested in this too. Does anybody have any ideas?

validates_uniqueness_of :username, :scope => :password

(this is an extra query. You could write a unique index and rescue the