Rails Active Support provides various extensions, utilities, and helpers. It provides a collection of utility classes and standard library extensions that are very useful.

Rails 5.1 introduced assert_no_changes and assert_changes making it easier to observe changes. Using ActiveSupport::TestCase#assert_no_changes we can easily assert that the result of evaluating an expression has not changed before and after calling the passed block.

To assert the expected change in the value of an object we can use assert_changes.

assert_changes -> { user.address } do
  user.update address: 'Miami'
end

assert_changes also supports from and to options.

assert_changes -> { user.address }, from: 'San Francisco', to: 'Miami' do
  user.update address: 'Miami'
end

Similarly, assert_no_changes allows us to assert a value that is expected to not change.

assert_no_changes -> { user.address } do
  user.update address: 'Miami'
end

We can also specify an error message with assert_no_changes.

assert_no_changes -> { user.address }, 'Expect the address to not change' do
  user.update address: 'Miami'
end

Before

assert_no_changes did not support the from option similar to assert_changes.

assert_no_changes -> { user.address } do
  user.update address: 'Miami'
end

However, Rails 7 has added from: option to ActiveSupport::TestCase#assert_no_changes, allowing us to assert on the initial value that is expected to not change.

Rails 7 onwards

Provides the optional from argument to specify the expected initial value.

assert_no_changes -> { user.address }, from: 'San Francisco' do
  user.update address: 'Miami'
end

Check out this pull request for more details.