Mongoid doesn't automatically cast strings to BSON::ObjectID

gistfile1.rb

# this doesn't work, because username_or_id is a string either way
def self.find_by_username_or_id(username_or_id) 	
  User.any_of({:username => username_or_id}, {:_id => username_or_id}).first 
  # raises "illegal ObjectID format" exception
end

gistfile2.rb

# this works because we cast valid object ids to BSON::ObjectID
def self.find_by_username_or_id(username_or_id)
  if BSON::ObjectID.legal?(username_or_id)
    User.first :conditions => {:_id => BSON::ObjectID(username_or_id)}
  else
    User.first :conditions => {:username => username_or_id}
  end
end