I was surprised to see that Minitest does not offer an
assert_not_changed method by default. Rails has a similar method to this called assert_difference but it does a different job. Here are two ways you can add this to your test suite.
One way to add it, is to get it using my gem rearmed which is a collection of helpful monkey patches for Rails and Ruby that are safely applied on opt-in basis.
Or if you don't use that gem just add this to your test helpers:
@str = 'test' assert_changed '@str' do @str = 'foobar' end assert_not_changed '@str' do @str = false end