added nicer failure reporting to #assert_difference to tell you the expression that failed rather than just the expected and actual values

Signed-off-by: Michael Koziarski <michael@koziarski.com>
[#1161 state:committed]

activesupport/lib/active_support/testing/core_ext/test/unit/assertions.rb

@@ -37,15 +37,18 @@ module Test
       #   end
       def assert_difference(expressions, difference = 1, message = nil, &block)
         expression_evaluations = Array(expressions).map do |expression|
-          lambda do
+          [expression, lambda do
             eval(expression, block.__send__(:binding))
-          end
+          end]
         end
 
-        original_values = expression_evaluations.inject([]) { |memo, expression| memo << expression.call }
+        original_values = expression_evaluations.inject([]) { |memo, expression| memo << expression[1].call }
         yield
         expression_evaluations.each_with_index do |expression, i|
-          assert_equal original_values[i] + difference, expression.call, message
+          full_message = ""
+          full_message << "#{message}.\n" if message
+          full_message << "<#{expression[0]}> was the expression that failed"
+          assert_equal original_values[i] + difference, expression[1].call, full_message
         end
       end
 

activesupport/test/test_test.rb

@@ -60,6 +60,24 @@ class AssertDifferenceTest < Test::Unit::TestCase
         @object.increment
       end
     end
+
+    def test_array_of_expressions_identify_failure
+      assert_difference ['@object.num', '1 + 1'] do
+        @object.increment
+      end
+      fail 'should not get to here'
+    rescue Test::Unit::AssertionFailedError => e
+      assert_equal "<1 + 1> was expression that failed.\n<3> expected but was\n<2>.", e.message
+    end
+
+    def test_array_of_expressions_identify_failure_when_message_provided
+      assert_difference ['@object.num', '1 + 1'], 1, 'something went wrong' do
+        @object.increment
+      end
+      fail 'should not get to here'
+    rescue Test::Unit::AssertionFailedError => e
+      assert_equal "something went wrong.\n<1 + 1> was expression that failed.\n<3> expected but was\n<2>.", e.message
+    end
   else
     def default_test; end
   end