find_sti_class

# File activerecord/lib/active_record/inheritance.rb, line 309
        def find_sti_class(type_name)
          type_name = base_class.type_for_attribute(inheritance_column).cast(type_name)
          subclass = sti_class_for(type_name)

          unless subclass == self || descendants.include?(subclass)
            raise SubclassNotFound, "Invalid single-table inheritance type: #{subclass.name} is not a subclass of #{name}"
          end

          subclass
        end