1 files changed, 15 insertions, 4 deletions
diff --git a/activesupport/lib/active_support/testing/assertions.rb b/activesupport/lib/active_support/testing/assertions.rb
index a4e0361a32..bfc2024529 100644
--- a/activesupport/lib/active_support/testing/assertions.rb
+++ b/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