find_sti_class

# File activerecord/lib/active_record/inheritance.rb, line 217
        def find_sti_class(type_name)
          type_name = base_class.type_for_attribute(inheritance_column).cast(type_name)
          subclass = begin
            if store_full_sti_class
              ActiveSupport::Dependencies.constantize(type_name)
            else
              compute_type(type_name)
            end
          rescue NameError
            raise SubclassNotFound,
              "The single-table inheritance mechanism failed to locate the subclass: '#{type_name}'. "                "This error is raised because the column '#{inheritance_column}' is reserved for storing the class in case of inheritance. "                "Please rename this column if you didn't intend it to be used for storing the inheritance class "                "or overwrite #{name}.inheritance_column to use another column for that information."
          end
          unless subclass == self || descendants.include?(subclass)
            raise SubclassNotFound, "Invalid single-table inheritance type: #{subclass.name} is not a subclass of #{name}"
          end
          subclass
        end