Sets the attribute used for single table inheritance to this class name if this is not the ActiveRecord::Base descendent. Considering the hierarchy Reply < Message < ActiveRecord::Base, this makes it possible to do Reply.new without having to set Reply = "Reply" yourself. No such attribute would be set for objects of the Message class in that example.
# File activerecord/lib/active_record/base.rb, line 2736 def ensure_proper_type unless self.class.descends_from_active_record? write_attribute(self.class.inheritance_column, self.class.sti_name) end end