assert_difference can take a callable piece of code rather than just evaling a string

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -576,11 +576,11 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
   end
 
   def test_polymorphic_counter_cache
-    tagging     = taggings(:welcome_general)
-    post = post = posts(:welcome)
-    comment     = comments(:greetings)
+    tagging = taggings(:welcome_general)
+    post    = posts(:welcome)
+    comment = comments(:greetings)
 
-    assert_difference 'post.reload.taggings_count', -1 do
+    assert_difference lambda { post.reload.taggings_count }, -1 do
       assert_difference 'comment.reload.taggings_count', +1 do
         tagging.taggable = comment
       end

activesupport/lib/active_support/testing/assertions.rb

@@ -29,6 +29,16 @@ module ActiveSupport
       #     post :create, :article => {...}
       #   end
       #
+      # A lambda or a list of lambdas can be passed in and evaluated:
+      #
+      #   assert_difference lambda { Article.count }, 2 do
+      #     post :create, :article => {...}
+      #   end
+      #
+      #   assert_difference [->{ Article.count }, ->{ Post.count }], 2 do
+      #     post :create, :article => {...}
+      #   end
+      #
       # A error message can be specified.
       #
       #   assert_difference 'Article.count', -1, "An Article should be destroyed" do
@@ -37,14 +47,15 @@ module ActiveSupport
       def assert_difference(expression, difference = 1, message = nil, &block)
         b = block.send(:binding)
         exps = Array.wrap(expression)
-        before = exps.map { |e| eval(e, b) }
+        before = exps.map { |e| e.respond_to?(:call) ? e.call : eval(e, b) }
 
         yield
 
         exps.each_with_index do |e, i|
-          error = "#{e.inspect} didn't change by #{difference}"
-          error = "#{message}.\n#{error}" if message
-          assert_equal(before[i] + difference, eval(e, b), error)
+          error  = "#{e.inspect} didn't change by #{difference}"
+          error  = "#{message}.\n#{error}" if message
+          actual = e.respond_to?(:call) ? e.call : eval(e, b)
+          assert_equal(before[i] + difference, actual, error)
         end
       end