Bring back assert_serializer

The `assert_serializer` test helper was removed at some point.

This patch brings back the `assert_serializer` test helper. This is the last
revision[1] that has the helper. The original helper was used as base.

[1]: https://github.com/rails-api/active_model_serializers/tree/610aeb2e9297fa31b8d561f0be9a4597f0258f8c

CHANGELOG.md

@@ -1,5 +1,6 @@
 ### 0.10.0
 
+  * adds `assert_serializer` test helper [@maurogeorge]
   * adds adapters pattern
   * adds support for `meta` and `meta_key` [@kurko]
   * adds method to override association [@kurko]

README.md

@@ -335,6 +335,34 @@ class PostSerializer < ActiveModel::Serializer
 end
 ```
 
+## Test helpers
+
+AMS provides a `assert_serializer` method to be used on your controller tests to
+assert that a specific serializer was used.
+
+```ruby
+class PostsControllerTest < ActionController::TestCase
+  test "should render post serializer" do
+    get :index
+    assert_serializer "PostSerializer"
+    #  # return a custom error message
+    #  assert_serializer "PostSerializer", "PostSerializer not rendered"
+    #
+    #  # assert that the instance of PostSerializer was rendered
+    #  assert_serializer PostSerializer
+    #
+    #  # assert that the "PostSerializer" serializer was rendered
+    #  assert_serializer :post_serializer
+    #
+    #  # assert that the rendered serializer starts with "Post"
+    #  assert_serializer %r{\APost.+\Z}
+    #
+    #  # assert that no serializer was rendered
+    #  assert_serializer nil
+  end
+end
+```
+
 ## Getting Help
 
 If you find a bug, please report an [Issue](https://github.com/rails-api/active_model_serializers/issues/new).

lib/action_controller/serialization_test_case.rb

@@ -0,0 +1,78 @@
+module ActionController
+  module SerializationAssertions
+    extend ActiveSupport::Concern
+
+    included do
+      setup :setup_serialization_subscriptions
+      teardown :teardown_serialization_subscriptions
+    end
+
+    # Asserts that the request was rendered with the appropriate serializers.
+    #
+    #  # assert that the "PostSerializer" serializer was rendered
+    #  assert_serializer "PostSerializer"
+    #
+    #  # return a custom error message
+    #  assert_serializer "PostSerializer", "PostSerializer not rendered"
+    #
+    #  # assert that the instance of PostSerializer was rendered
+    #  assert_serializer PostSerializer
+    #
+    #  # assert that the "PostSerializer" serializer was rendered
+    #  assert_serializer :post_serializer
+    #
+    #  # assert that the rendered serializer starts with "Post"
+    #  assert_serializer %r{\APost.+\Z}
+    #
+    #  # assert that no serializer was rendered
+    #  assert_serializer nil
+    #
+    def assert_serializer(expectation, message = nil)
+      # Force body to be read in case the template is being streamed.
+      response.body
+
+      msg = message || "expecting <#{expectation.inspect}> but rendering with <#{serializers}>"
+
+      matches_serializer = case expectation
+                           when ->(exp) { exp.kind_of?(Class) && exp < ActiveModel::Serializer }
+                             serializers.include?(expectation.name)
+                           when Symbol
+                             expectation = expectation.to_s.camelize
+                             serializers.include?(expectation)
+                           when String
+                             !expectation.empty? && serializers.include?(expectation)
+                           when Regexp
+                             serializers.any? do |serializer|
+                               serializer.match(expectation)
+                             end
+                           when NilClass
+                             serializers.blank?
+                           else
+                             raise ArgumentError, "assert_serializer only accepts a String, Symbol, Regexp, ActiveModel::Serializer, or nil"
+                           end
+      assert(matches_serializer, msg)
+    end
+
+    private
+
+    ActiveModelSerializers.silence_warnings do
+    attr_reader :serializers
+    end
+
+    def setup_serialization_subscriptions
+      @serializers = []
+      ActiveSupport::Notifications.subscribe(event_name) do |name, start, finish, id, payload|
+        serializer = payload[:serializer]
+        serializers << serializer
+      end
+    end
+
+    def teardown_serialization_subscriptions
+      ActiveSupport::Notifications.unsubscribe(event_name)
+    end
+
+    def event_name
+      "serialize.active_model_serializers"
+    end
+  end
+end

lib/active_model/serializable_resource.rb

@@ -19,6 +19,7 @@ def initialize(resource, options = {})
     # returns the contents of the block
     def self.serialize(resource, options = {})
       serializable_resource = SerializableResource.new(resource, options)
+      serializable_resource.instrument
       if block_given?
         yield serializable_resource
       else
@@ -74,6 +75,14 @@ def serializer?
       use_adapter? && !!(serializer)
     end
 
+    def instrument
+      if serializer?
+        event_name = "serialize.active_model_serializers"
+        payload = { serializer: serializer.name }
+        ActiveSupport::Notifications.instrument(event_name, payload)
+      end
+    end
+
     private
 
     ActiveModelSerializers.silence_warnings do

lib/active_model_serializers.rb

@@ -19,11 +19,13 @@ def silence_warnings
 require 'active_model/serializer/version'
 
 require 'action_controller/serialization'
+require 'action_controller/serialization_test_case'
 ActiveSupport.on_load(:action_controller) do
   include ::ActionController::Serialization
   ActionDispatch::Reloader.to_prepare do
     ActiveModel::Serializer.serializers_cache.clear
   end
+  ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
 end
 
 require 'active_model/serializer/railtie'

test/action_controller/serialization_test_case_test.rb

@@ -0,0 +1,70 @@
+require 'test_helper'
+
+module ActionController
+  module SerializationsAssertions
+    class RenderSerializerTest < ActionController::TestCase
+      class MyController < ActionController::Base
+        def render_using_serializer
+          render json: Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
+        end
+
+        def render_text
+          render text: 'ok'
+        end
+
+        def render_template
+          prepend_view_path "./test/fixtures"
+          render template: "template"
+        end
+      end
+
+      tests MyController
+
+      def test_supports_specifying_serializers_with_a_serializer_class
+        get :render_using_serializer
+        assert_serializer ProfileSerializer
+      end
+
+      def test_supports_specifying_serializers_with_a_regexp
+        get :render_using_serializer
+        assert_serializer %r{\AProfile.+\Z}
+      end
+
+      def test_supports_specifying_serializers_with_a_string
+        get :render_using_serializer
+        assert_serializer 'ProfileSerializer'
+      end
+
+      def test_supports_specifying_serializers_with_a_symbol
+        get :render_using_serializer
+        assert_serializer :profile_serializer
+      end
+
+      def test_supports_specifying_serializers_with_a_nil
+        get :render_text
+        assert_serializer nil
+      end
+
+      def test_raises_descriptive_error_message_when_serializer_was_not_rendered
+        get :render_using_serializer
+        e = assert_raise ActiveSupport::TestCase::Assertion do
+          assert_serializer 'PostSerializer'
+        end
+        assert_match 'expecting <"PostSerializer"> but rendering with <["ProfileSerializer"]>', e.message
+      end
+
+      def test_raises_argument_error_when_asserting_with_invalid_object
+        get :render_using_serializer
+        e = assert_raise ArgumentError do
+          assert_serializer Hash
+        end
+        assert_match 'assert_serializer only accepts a String, Symbol, Regexp, ActiveModel::Serializer, or nil', e.message
+      end
+
+      def test_does_not_overwrite_notification_subscriptions
+        get :render_template
+        assert_template "template"
+      end
+    end
+  end
+end

test/fixtures/template.html.erb

@@ -0,0 +1 @@
+<p>Hello.</p>