instance method emit_warning_if_needed

# File activerecord/lib/active_record/attribute_methods/dirty.rb, line 249
        def emit_warning_if_needed(method_name, new_method_name)
          unless mutation_tracker.equal?(mutations_from_database)
            ActiveSupport::Deprecation.warn(<<-EOW.squish)
              The behavior of `#{method_name}` inside of after callbacks will
              be changing in the next version of Rails. The new return value will reflect the
              behavior of calling the method after `save` returned (e.g. the opposite of what
              it returns now). To maintain the current behavior, use `#{new_method_name}`
              instead.
            EOW
          end
        end