you could also do

    @name = Support.validate_db_name(name) unless name == '$external'

Also, where is the Mongo module being included?  Maybe we want to simplify this line too:
https://github.com/brandonblack/mongo-ruby-driver/blob/52bf2b80e391e4214bd847241484470db4b0b0fc/lib/mongo/db.rb#L504