I have found a solution for the problem. I’m not quite satisfied with it (I’d rather have a way to specify this in the initializer), but it works for now. In the user model I added the following method:
def self.find_for_database_authentication(conditions={})
find_by(username: conditions[:email]) || find_by(email: conditions[:email])
end
As @sguha and @Chetan have pointed out, another great resource is available on the official devise wiki.