Test::Unit assert_serializer implemented

So you can assert specific serializer to be used.

lib/action_controller/serialization_test_case.rb

@@ -0,0 +1,72 @@
+module ActionController
+  module SerializationAssertions
+    extend ActiveSupport::Concern
+
+    included do
+      setup :setup_subscriptions
+      teardown :teardown_subscriptions
+    end
+
+    def setup_subscriptions
+      @serializers = Hash.new(0)
+
+      ActiveSupport::Notifications.subscribe("!serialize.active_model_serializers") do |name, start, finish, id, payload|
+        serializer = payload[:serializer]
+        @serializers[serializer] += 1
+      end
+    end
+
+    def teardown_subscriptions
+      ActiveSupport::Notifications.unsubscribe("!serialize.active_model_serializers")
+    end
+
+    def process(*args)
+      @serializers = Hash.new(0)
+      super
+    end
+
+    # Asserts that the request was rendered with the appropriate serializers.
+    #
+    #   # assert that the "PostSerializer" serializer was rendered
+    #   assert_serializer "PostSerializer"
+    #
+    #   # assert that the exact serializer "Post" was rendered
+    #   assert_serializer %r{\APost.+\Z}
+    #
+    #   # assert that no serializer was rendered
+    #   assert_serializer nil
+    #
+    def assert_serializer(options = {}, message = nil)
+      # Force body to be read in case the template is being streamed.
+      response.body
+
+      case options
+      when NilClass, Regexp, String, Symbol
+        rendered = @serializers
+        msg = message || "expecting <#{options.inspect}> but rendering with <#{rendered.keys}>"
+
+        matches_serializer =
+          case options
+          when Symbol
+            options = options.to_s.camelize
+            rendered.any? do |serializer, count|
+              serializer == options
+            end
+          when String
+            !options.empty? && rendered.any? do |serializer, count|
+              serializer == options
+            end
+          when Regexp
+            rendered.any? do |serializer, count|
+              serializer.match(options)
+            end
+          when NilClass
+            rendered.blank?
+          end
+        assert matches_serializer, msg
+      else
+        raise ArgumentError, "assert_serializer only accepts a String, Symbol, Regexp, or nil"
+      end
+    end
+  end
+end