From 21aea0db553ae935858bb14e4f6b3c377bc2449d Mon Sep 17 00:00:00 2001
From: George Claghorn <george.claghorn@gmail.com>
Date: Sun, 23 May 2021 16:22:43 -0400
Subject: [PATCH] Add from: option to ActiveSupport::TestCase#assert_no_changes

Permit asserting on the initial value that is expected not to change.
---
 activesupport/CHANGELOG.md                    | 16 ++++++++--
 .../lib/active_support/testing/assertions.rb  | 16 +++++++++-
 activesupport/test/test_case_test.rb          | 31 +++++++++++++++++++
 3 files changed, 60 insertions(+), 3 deletions(-)

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index ca26893537..86dfc6ce67 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,15 @@
+*   The `from:` option is added to `ActiveSupport::TestCase#assert_no_changes`.
+
+    It permits asserting on the initial value that is expected not to change.
+
+    ```ruby
+    assert_no_changes -> { Status.all_good? }, from: true do
+      post :create, params: { status: { ok: true } }
+    end
+    ```
+  
+    *George Claghorn*
+
 *   Deprecate `ActiveSupport::SafeBuffer`'s incorrect implicit conversion of objects into string.
 
     Except for a few methods like `String#%`, objects must implement `#to_str`
@@ -73,9 +85,9 @@
     Ruby requires an initializer for non-numeric type as per examples below:
 
     ```ruby
-    %w[foo bar].sum('') 
+    %w[foo bar].sum('')
     # instead of %w[foo bar].sum
-    
+
     [[1, 2], [3, 4, 5]].sum([])
     #instead of [[1, 2], [3, 4, 5]].sum
     ```
diff --git a/activesupport/lib/active_support/testing/assertions.rb b/activesupport/lib/active_support/testing/assertions.rb
index d564cad808..437fc07909 100644
--- a/activesupport/lib/active_support/testing/assertions.rb
+++ b/activesupport/lib/active_support/testing/assertions.rb
@@ -207,16 +207,30 @@ module ActiveSupport
       #     post :create, params: { status: { ok: true } }
       #   end
       #
+      # Provide the optional keyword argument :from to specify the expected
+      # initial value.
+      #
+      #   assert_no_changes -> { Status.all_good? }, from: true do
+      #     post :create, params: { status: { ok: true } }
+      #   end
+      #
       # An error message can be specified.
       #
       #   assert_no_changes -> { Status.all_good? }, 'Expected the status to be good' do
       #     post :create, params: { status: { ok: false } }
       #   end
-      def assert_no_changes(expression, message = nil, &block)
+      def assert_no_changes(expression, message = nil, from: UNTRACKED, &block)
         exp = expression.respond_to?(:call) ? expression : -> { eval(expression.to_s, block.binding) }
 
         before = exp.call
         retval = assert_nothing_raised(&block)
+
+        unless from == UNTRACKED
+          error = "Expected initial value of #{from.inspect}"
+          error = "#{message}.\n#{error}" if message
+          assert from === before, error
+        end
+
         after = exp.call
 
         error = "#{expression.inspect} changed"
diff --git a/activesupport/test/test_case_test.rb b/activesupport/test/test_case_test.rb
index e6342e8382..6339792ecf 100644
--- a/activesupport/test/test_case_test.rb
+++ b/activesupport/test/test_case_test.rb
@@ -286,6 +286,37 @@ class AssertionsTest < ActiveSupport::TestCase
     end
   end
 
+  def test_assert_no_changes_with_from_option
+    assert_no_changes "@object.num", from: 0 do
+      # ...
+    end
+  end
+
+  def test_assert_no_changes_with_from_option_with_wrong_value
+    assert_raises Minitest::Assertion do
+      assert_no_changes "@object.num", from: -1 do
+        # ...
+      end
+    end
+  end
+
+  def test_assert_no_changes_with_from_option_with_nil
+    error = assert_raises Minitest::Assertion do
+      assert_no_changes "@object.num", from: nil do
+        @object.increment
+      end
+    end
+    assert_equal "Expected initial value of nil", error.message
+  end
+
+  def test_assert_no_changes_with_from_and_case_operator
+    token = SecureRandom.hex
+
+    assert_no_changes -> { token }, from: /\w{32}/ do
+      # ...
+    end
+  end
+
   def test_assert_no_changes_with_message
     error = assert_raises Minitest::Assertion do
       assert_no_changes "@object.num", "@object.num should not change" do