Support hash as first argument in `assert_difference`. (#31600)

* Support hash as first argument for `assert_difference`.

This allows to specify multiple numeric differences in the same assertion.
Example:

    assert_difference 'Article.count' => 1, 'Notification.count' => 2 do
      # post :create, params: { article: {...} }
    end

* Support error message when passing a hash as a first parameter

* Format CHANGELOG properly

[Julien Meichelbeck + Rafael Mendonça França]

3 files changed, 57 insertions, 7 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index a21a817fbb..acff6367f2 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Support hash as first argument in `assert_difference`. This allows to specify multiple
+    numeric differences in the same assertion.
+
+        assert_difference ->{ Article.count } => 1, ->{ Post.count } => 2
+
+    *Julien Meichelbeck*
+
 *   Add missing instrumentation for `read_multi` in `ActiveSupport::Cache::Store`.
 
     *Ignatius Reza Lesmana*
diff --git a/activesupport/lib/active_support/testing/assertions.rb b/activesupport/lib/active_support/testing/assertions.rb
index 6f69c48674..a891ff616d 100644
--- a/activesupport/lib/active_support/testing/assertions.rb
+++ b/activesupport/lib/active_support/testing/assertions.rb
@@ -58,6 +58,12 @@ module ActiveSupport
       #     post :create, params: { article: {...} }
       #   end
       #
+      # A hash of expressions/numeric differences can also be passed in and evaluated.
+      #
+      #   assert_difference ->{ Article.count } => 1, ->{ Notification.count } => 2 do
+      #     post :create, params: { article: {...} }
+      #   end
+      #
       # A lambda or a list of lambdas can be passed in and evaluated:
       #
       #   assert_difference ->{ Article.count }, 2 do
@@ -73,20 +79,28 @@ module ActiveSupport
       #   assert_difference 'Article.count', -1, 'An Article should be destroyed' do
       #     post :delete, params: { id: ... }
       #   end
-      def assert_difference(expression, difference = 1, message = nil, &block)
-        expressions = Array(expression)
-
-        exps = expressions.map { |e|
+      def assert_difference(expression, *args, &block)
+        expressions =
+          if expression.is_a?(Hash)
+            message = args[0]
+            expression
+          else
+            difference = args[0] || 1
+            message = args[1]
+            Hash[Array(expression).map { |e| [e, difference] }]
+          end
+
+        exps = expressions.keys.map { |e|
           e.respond_to?(:call) ? e : lambda { eval(e, block.binding) }
         }
         before = exps.map(&:call)
 
         retval = yield
 
-        expressions.zip(exps).each_with_index do |(code, e), i|
-          error  = "#{code.inspect} didn't change by #{difference}"
+        expressions.zip(exps, before) do |(code, diff), exp, before_value|
+          error  = "#{code.inspect} didn't change by #{diff}"
           error  = "#{message}.\n#{error}" if message
-          assert_equal(before[i] + difference, e.call, error)
+          assert_equal(before_value + diff, exp.call, error)
         end
 
         retval
diff --git a/activesupport/test/test_case_test.rb b/activesupport/test/test_case_test.rb
index 79b75a001a..eced622137 100644
--- a/activesupport/test/test_case_test.rb
+++ b/activesupport/test/test_case_test.rb
@@ -115,6 +115,35 @@ class AssertDifferenceTest < ActiveSupport::TestCase
     end
   end
 
+  def test_hash_of_expressions
+    assert_difference "@object.num" => 1, "@object.num + 1" => 1 do
+      @object.increment
+    end
+  end
+
+  def test_hash_of_expressions_with_message
+    error = assert_raises Minitest::Assertion do
+      assert_difference({ "@object.num" => 0 }, "Object Changed") do
+        @object.increment
+      end
+    end
+    assert_equal "Object Changed.\n\"@object.num\" didn't change by 0.\nExpected: 0\n  Actual: 1", error.message
+  end
+
+  def test_hash_of_lambda_expressions
+    assert_difference -> { @object.num } => 1, -> { @object.num + 1 } => 1 do
+      @object.increment
+    end
+  end
+
+  def test_hash_of_expressions_identify_failure
+    assert_raises(Minitest::Assertion) do
+      assert_difference "@object.num" => 1, "1 + 1" => 1 do
+        @object.increment
+      end
+    end
+  end
+
   def test_assert_changes_pass
     assert_changes "@object.num" do
       @object.increment