Merge pull request #31011 from danielma/dma/assert-changes-with-to-sh…

…ould-still-assert-change

`assert_changes` should always assert some change

activesupport/CHANGELOG.md

@@ -1,10 +1,16 @@
+*   `assert_changes` will always assert that the expression changes,
+    regardless of `from:` and `to:` argument combinations.
+
+    *Daniel Ma*
+
 *   Allow the hash function used to generate non-sensitive digests, such as the
     ETag header, to be specified with `config.active_support.hash_digest_class`.
 
     The object provided must respond to `#hexdigest`, e.g. `Digest::SHA1`.
 
     *Dmitri Dolguikh*
-
+
+
 ## Rails 5.2.0.beta2 (November 28, 2017) ##
 
 *   No changes.

activesupport/lib/active_support/testing/assertions.rb

@@ -156,11 +156,12 @@ def assert_changes(expression, message = nil, from: UNTRACKED, to: UNTRACKED, &b
 
         after = exp.call
 
-        if to == UNTRACKED
-          error = "#{expression.inspect} didn't change"
-          error = "#{message}.\n#{error}" if message
-          assert before != after, error
-        else
+        error = "#{expression.inspect} didn't change"
+        error = "#{error}. It was already #{to}" if before == to
+        error = "#{message}.\n#{error}" if message
+        assert before != after, error
+
+        unless to == UNTRACKED
           error = "#{expression.inspect} didn't change to #{to}"
           error = "#{message}.\n#{error}" if message
           assert to === after, error

activesupport/test/test_case_test.rb

@@ -156,6 +156,16 @@ def test_assert_changes_with_to_option
     end
   end
 
+  def test_assert_changes_with_to_option_but_no_change_has_special_message
+    error = assert_raises Minitest::Assertion do
+      assert_changes "@object.num", to: 0 do
+        # no changes
+      end
+    end
+
+    assert_equal "\"@object.num\" didn't change. It was already 0", error.message
+  end
+
   def test_assert_changes_with_wrong_to_option
     assert_raises Minitest::Assertion do
       assert_changes "@object.num", to: 2 do
@@ -218,6 +228,7 @@ def test_assert_changes_with_to_and_from_and_case_operator
   def test_assert_changes_with_message
     error = assert_raises Minitest::Assertion do
       assert_changes "@object.num", "@object.num should 1", to: 1 do
+        @object.decrement
       end
     end