Single row update without a select or knowing id

I was looking for a single row update that doesn't use a select. The closest available in Rails in Model.update() but it required an ID, which I don't have. What I do have is the attributes of some of the other fields. I was hoping to use something like this for example:

Model.update({ :email => 'test@test.com' }, { :conditions => { :login => 'someone' })

The resulting SQL would look like this of course:

UPDATE model SET email = 'test@test.com' WHERE login = 'someone';

However, there doesn't seem to be a way to do this in Rails at the moment, and the only way to do it is to run a find_by_login and update_attribute call on the result, but as you know, that makes two queries.

Any ideas when a single update method that doesn't require an id, might be available?