1 files changed, 89 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/testing/assertions.rb b/activesupport/lib/active_support/testing/assertions.rb
index ad83638572..7770aa8006 100644
--- a/activesupport/lib/active_support/testing/assertions.rb
+++ b/activesupport/lib/active_support/testing/assertions.rb
@@ -1,6 +1,8 @@
 module ActiveSupport
   module Testing
     module Assertions
+      UNTRACKED = Object.new # :nodoc:
+
       # Asserts that an expression is not truthy. Passes if <tt>object</tt> is
       # +nil+ or +false+. "Truthy" means "considered true in a conditional"
       # like <tt>if foo</tt>.
@@ -92,6 +94,93 @@ module ActiveSupport
       def assert_no_difference(expression, message = nil, &block)
         assert_difference expression, 0, message, &block
       end
+
+      # Assertion that the result of evaluating an expression is changed before
+      # and after invoking the passed in block.
+      #
+      #   assert_changes 'Status.all_good?' do
+      #     post :create, params: { status: { ok: false } }
+      #   end
+      #
+      # You can pass the block as a string to be evaluated in the context of
+      # the block. A lambda can be passed for the block as well.
+      #
+      #   assert_changes -> { Status.all_good? } do
+      #     post :create, params: { status: { ok: false } }
+      #   end
+      #
+      # The assertion is useful to test side effects. The passed block can be
+      # anything that can be converted to string with #to_s.
+      #
+      #   assert_changes :@object do
+      #     @object = 42
+      #   end
+      #
+      # The keyword arguments :from and :to can be given to specify the
+      # expected initial value and the expected value after the block was
+      # executed.
+      #
+      #   assert_changes :@object, from: nil, to: :foo do
+      #     @object = :foo
+      #   end
+      #
+      # An error message can be specified.
+      #
+      #   assert_changes -> { Status.all_good? }, 'Expected the status to be bad' do
+      #     post :create, params: { status: { incident: true } }
+      #   end
+      def assert_changes(expression, message = nil, from: UNTRACKED, to: UNTRACKED, &block)
+        exp = expression.respond_to?(:call) ? expression : -> { eval(expression.to_s, block.binding) }
+
+        before = exp.call
+        retval = yield
+
+        unless from == UNTRACKED
+          error = "#{expression.inspect} isn't #{from}"
+          error = "#{message}.\n#{error}" if message
+          assert from === before, error
+        end
+
+        after = exp.call
+
+        if to == UNTRACKED
+          error = "#{expression.inspect} didn't changed"
+          error = "#{message}.\n#{error}" if message
+          assert_not_equal before, after, error
+        else
+          message = "#{expression.inspect} didn't change to #{to}"
+          error = "#{message}.\n#{error}" if message
+          assert to === after, error
+        end
+
+        retval
+      end
+
+      # Assertion that the result of evaluating an expression is changed before
+      # and after invoking the passed in block.
+      #
+      #   assert_no_changes 'Status.all_good?' 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)
+        exp = expression.respond_to?(:call) ? expression : -> { eval(expression.to_s, block.binding) }
+
+        before = exp.call
+        retval = yield
+        after = exp.call
+
+        error = "#{expression.inspect} did change to #{after}"
+        error = "#{message}.\n#{error}" if message
+        assert_equal before, after, error
+
+        retval
+      end
     end
   end
 end