def __compat_set_attribute_was(name, previous)
      if respond_to?(:set_attribute_was, true)
        # Rails 4.2+
        set_attribute_was(name, previous)
      else
        # Rails < 4.2
        changed_attributes[name] = previous
      end
    end
def __compat_set_attribute_was(name, previous)
      if respond_to?(:set_attribute_was, true)
        # Rails 4.2+
        set_attribute_was(name, previous)
      else
        # Rails < 4.2
        changed_attributes[name] = previous
      end
    end
Rails 4.2 adds "set_attribute_was" which must be used, so we'll
 check for it.