Bring back assert_serializer for controller testing

The `assert_serializer` test helper was added in 0.9.0.apha1[1],
and was not included in 0.10.

This patch brings back the `assert_serializer` test helper. This is the last
revision[2] that has the helper. The original helper was used as base.

[1]: https://github.com/rails-api/active_model_serializers/pull/596
[2]: 610aeb2e92

- Create the AssertSerializer

- Use the Test namespace

- Make the tests pass on the Rails master
  - Rails 5 does not include `assert_template` but we need this on the tests of
the helper.
  - This add the `rails-controller-testing` to keep support on `assert_template`.

- Only load test helpers in the test environment

CHANGELOG.md

@@ -45,6 +45,7 @@ Features:
     CollectionSerializer for clarity, add ActiveModelSerializers.config.collection_serializer (@bf4)
 - [#1295](https://github.com/rails-api/active_model_serializers/pull/1295) Add config `serializer_lookup_enabled` that,
   when disabled, requires serializers to explicitly specified. (@trek)
+- [#1099](https://github.com/rails-api/active_model_serializers/pull/1099) Adds `assert_serializer` test helper (@maurogeorge)
 
 Fixes:
 - [#1239](https://github.com/rails-api/active_model_serializers/pull/1239) Fix duplicates in JSON API compound documents (@beauby)

Gemfile

@@ -12,6 +12,7 @@ version = ENV['RAILS_VERSION'] || '4.2'
 if version == 'master'
   gem 'rack', github: 'rack/rack'
   gem 'arel', github: 'rails/arel'
+  gem 'rails-controller-testing', github: 'rails/rails-controller-testing'
   git 'https://github.com/rails/rails.git' do
     gem 'railties'
     gem 'activesupport'

docs/README.md

@@ -22,6 +22,7 @@ This is the documentation of ActiveModelSerializers, it's focused on the **0.10.
 - [How to add root key](howto/add_root_key.md)
 - [How to add pagination links](howto/add_pagination_links.md)
 - [Using ActiveModelSerializers Outside Of Controllers](howto/outside_controller_use.md)
+- [Testing ActiveModelSerializers](howto/test.md)
 
 ## Integrations
 

docs/howto/test.md

@@ -0,0 +1,29 @@
+# How to test
+
+## Test helpers
+
+ActiveModelSerializers 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
+```

lib/active_model/serializer/railtie.rb

@@ -19,5 +19,9 @@ module ActiveModel
       app.load_generators
       require 'generators/serializer/resource_override'
     end
+
+    if Rails.env.test?
+      ActionController::TestCase.send(:include, ActiveModelSerializers::Test::Serializer)
+    end
   end
 end

lib/active_model_serializers.rb

@@ -14,6 +14,7 @@ module ActiveModelSerializers
   autoload :Callbacks
   autoload :Deserialization
   autoload :Logging
+  autoload :Test
 end
 
 require 'active_model/serializer'

lib/active_model_serializers/test.rb

@@ -0,0 +1,5 @@
+module ActiveModelSerializers
+  module Test
+    autoload :Serializer, 'active_model_serializers/test/serializer'
+  end
+end

lib/active_model_serializers/test/serializer.rb

@@ -0,0 +1,122 @@
+module ActiveModelSerializers
+  module Test
+    module Serializer
+      extend ActiveSupport::Concern
+
+      included do
+        setup :setup_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)
+        @assert_serializer.expectation = expectation
+        @assert_serializer.message = message
+        @assert_serializer.response = response
+        assert(@assert_serializer.matches?, @assert_serializer.message)
+      end
+
+      class AssertSerializer
+        attr_reader :serializers, :message
+        attr_accessor :response, :expectation
+
+        def initialize
+          @serializers = []
+        end
+
+        def message=(message)
+          @message = message || "expecting <#{expectation.inspect}> but rendering with <#{serializers}>"
+        end
+
+        def matches?
+          # Force body to be read in case the template is being streamed.
+          response.body
+
+          case expectation
+          when a_serializer?
+            matches_class?
+          when Symbol
+            matches_symbol?
+          when String
+            matches_string?
+          when Regexp
+            matches_regexp?
+          when NilClass
+            matches_nil?
+          else
+            fail ArgumentError, 'assert_serializer only accepts a String, Symbol, Regexp, ActiveModel::Serializer, or nil'
+          end
+        end
+
+        def subscribe
+          ActiveSupport::Notifications.subscribe(event_name) do |_name, _start, _finish, _id, payload|
+            serializer = payload[:serializer].name
+            serializers << serializer
+          end
+        end
+
+        def unsubscribe
+          ActiveSupport::Notifications.unsubscribe(event_name)
+        end
+
+        private
+
+        def matches_class?
+          serializers.include?(expectation.name)
+        end
+
+        def matches_symbol?
+          camelize_expectation = expectation.to_s.camelize
+          serializers.include?(camelize_expectation)
+        end
+
+        def matches_string?
+          !expectation.empty? && serializers.include?(expectation)
+        end
+
+        def matches_regexp?
+          serializers.any? do |serializer|
+            serializer.match(expectation)
+          end
+        end
+
+        def matches_nil?
+          serializers.blank?
+        end
+
+        def a_serializer?
+          ->(exp) { exp.is_a?(Class) && exp < ActiveModel::Serializer }
+        end
+
+        def event_name
+          'render.active_model_serializers'
+        end
+      end
+
+      private
+
+      def setup_serialization_subscriptions
+        @assert_serializer = AssertSerializer.new
+        @assert_serializer.subscribe
+      end
+    end
+  end
+end

test/active_model_serializers/test/serializer_test.rb

@@ -0,0 +1,73 @@
+require 'test_helper'
+require 'rails-controller-testing' if Rails::VERSION::MAJOR >= 5
+
+module ActiveModelSerializers
+  module Test
+    class SerializerTest < ActionController::TestCase
+      include ActiveModelSerializers::Test::Serializer
+
+      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(/\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>