Adding tests.

lib/active_model/serializable.rb

@@ -2,7 +2,7 @@ require 'active_model/serializable/utils'
 
 module ActiveModel
   module Serializable
-    INSTRUMENTATION_KEY = "!serialize.active_model_serializers".freeze
+    INSTRUMENTATION_KEY = '!serialize.active_model_serializers'.freeze
 
     def self.included(base)
       base.extend Utils

test/unit/active_model/serilizable_test.rb

@@ -0,0 +1,50 @@
+require 'test_helper'
+
+module ActiveModel
+  class SerializableTest
+    class InstrumentationTest < Minitest::Test
+      def setup
+        @events = []
+
+        @subscriber = ActiveSupport::Notifications.subscribe('!serialize.active_model_serializers') do |name, start, finish, id, payload|
+          @events << { name: name, serializer: payload[:serializer] }
+        end
+      end
+
+      def teardown
+        ActiveSupport::Notifications.unsubscribe(@subscriber) if defined?(@subscriber)
+      end
+
+      def test_instruments_default_serializer
+        DefaultSerializer.new(1).as_json
+
+        assert_equal [{ name: '!serialize.active_model_serializers', serializer: 'ActiveModel::DefaultSerializer' }], @events
+      end
+
+      def test_instruments_serializer
+        profile = Profile.new(name: 'Name 1', description: 'Description 1', comments: 'Comments 1')
+        serializer = ProfileSerializer.new(profile)
+
+        serializer.as_json
+
+        assert_equal [{ name: '!serialize.active_model_serializers', serializer: 'ProfileSerializer' }], @events
+      end
+
+      def test_instruments_array_serializer
+        profiles = [
+          Profile.new(name: 'Name 1', description: 'Description 1', comments: 'Comments 1'),
+          Profile.new(name: 'Name 2', description: 'Description 2', comments: 'Comments 2')
+        ]
+        serializer = ArraySerializer.new(profiles)
+
+        serializer.as_json
+
+        assert_equal [
+          { name: '!serialize.active_model_serializers', serializer: 'ProfileSerializer' },
+          { name: '!serialize.active_model_serializers', serializer: 'ProfileSerializer' },
+          { name: '!serialize.active_model_serializers', serializer: 'ActiveModel::ArraySerializer' }
+        ], @events
+      end
+    end
+  end
+end