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In default configuration (with core extensions loaded) conflicts with current version of ActiveRecord because of https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/calculations.rb#L296.
I consider that a bug in ActiveRecord. Sequel has been using the as method for over 4 years, so you should file the bug with them and get them to use something beyond a simple respond_to?(:as) check. If you want to work around the issue in Sequel, undef_method is your friend.