Only call railtie when Rails is defined; assume controller loaded

Isolated Testing

- Rake test inspired by https://github.com/rails/rails/blob/v5.0.0.beta1/activejob/Rakefile
- Isolated unit inspired by
  - https://github.com/rails/rails/blob/v5.0.0.beta1/railties/test/isolation/abstract_unit.rb
  - https://github.com/rails/rails/blob/v5.0.0.beta1/activemodel/test/cases/railtie_test.rb

Misc

- Turns out `mattr_accessor(:logger) {
  ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(STDOUT)) }`
  was always nil until the Railtie was loaded, since mattr_accessor
  block defaults don't really work on modules, but on the classes that
  include them.
- Commented on important on Rails being required first for caching to
  work.
- In isolated tests, `active_support/core_ext/object/with_options` is required.

test/active_model_serializers/railtie_test_isolated.rb

@@ -0,0 +1,69 @@
+# Execute this test in isolation
+require 'support/isolated_unit'
+
+class RailtieTest < ActiveSupport::TestCase
+  include ActiveSupport::Testing::Isolation
+
+  class WithRails < RailtieTest
+    setup do
+      require 'rails'
+      require 'active_model_serializers'
+      make_basic_app
+    end
+
+    test 'mixes ActionController::Serialization into ActionController::Base' do
+      assert ActionController.const_defined?(:Serialization),
+        "ActionController::Serialization should be defined, but isn't"
+      assert ::ActionController::Base.included_modules.include?(::ActionController::Serialization),
+        "ActionController::Serialization should be included in ActionController::Base, but isn't"
+    end
+
+    test 'sets the ActiveModelSerializers.logger to Rails.logger' do
+      refute_nil Rails.logger
+      refute_nil ActiveModelSerializers.logger
+      assert_equal Rails.logger, ActiveModelSerializers.logger
+    end
+
+    test 'it is configured for caching' do
+      assert_equal ActionController::Base.cache_store, ActiveModelSerializers.config.cache_store
+      assert_equal Rails.configuration.action_controller.perform_caching, ActiveModelSerializers.config.perform_caching
+    end
+
+    test 'it runs the load hook' do
+      loaded = false
+      ActiveSupport.on_load(:active_model_serializers) { loaded = true }
+      assert loaded
+    end
+  end
+
+  class WithoutRails < RailtieTest
+    setup do
+      require 'active_model_serializers'
+      make_basic_app
+    end
+
+    test 'does not mix ActionController::Serialization into ActionController::Base' do
+      refute ActionController.const_defined?(:Serialization),
+        'ActionController::Serialization should not be defined, but is'
+    end
+
+    test 'has its own logger at ActiveModelSerializers.logger' do
+      refute_nil Rails.logger
+      refute_nil ActiveModelSerializers.logger
+      refute_equal Rails.logger, ActiveModelSerializers.logger
+    end
+
+    test 'it is not configured for caching' do
+      refute_nil ActionController::Base.cache_store
+      assert_nil ActiveModelSerializers.config.cache_store
+      refute Rails.configuration.action_controller.perform_caching
+      refute ActiveModelSerializers.config.perform_caching
+    end
+
+    test "it hasn't run the load hook" do
+      loaded = false
+      ActiveSupport.on_load(:active_model_serializers) { loaded = true }
+      refute loaded
+    end
+  end
+end

test/support/isolated_unit.rb

@@ -0,0 +1,77 @@
+# https://github.com/rails/rails/blob/v5.0.0.beta1/railties/test/isolation/abstract_unit.rb
+
+# Usage Example:
+#
+# require 'support/isolated_unit'
+#
+# class RailtieTest < ActiveSupport::TestCase
+#   include ActiveSupport::Testing::Isolation
+#
+#   class WithRailsDefinedOnLoad < RailtieTest
+#     setup do
+#       require 'rails'
+#       require 'active_model_serializers'
+#       make_basic_app
+#     end
+#
+#     # some tests
+#   end
+#
+#   class WithoutRailsDefinedOnLoad < RailtieTest
+#     setup do
+#       require 'active_model_serializers'
+#       make_basic_app
+#     end
+#
+#     # some tests
+#   end
+# end
+#
+# Note:
+# It is important to keep this file as light as possible
+# the goal for tests that require this is to test booting up
+# rails from an empty state, so anything added here could
+# hide potential failures
+#
+# It is also good to know what is the bare minimum to get
+# Rails booted up.
+require 'bundler/setup' unless defined?(Bundler)
+require 'active_support'
+require 'active_support/core_ext/string/access'
+
+# These files do not require any others and are needed
+# to run the tests
+require 'active_support/testing/autorun'
+require 'active_support/testing/isolation'
+
+module TestHelpers
+  module Generation
+    # Make a very basic app, without creating the whole directory structure.
+    # Is faster and simpler than generating a Rails app in a temp directory
+    def make_basic_app
+      require 'rails'
+      require 'action_controller/railtie'
+
+      @app = Class.new(Rails::Application) do
+        config.eager_load = false
+        config.session_store :cookie_store, key: '_myapp_session'
+        config.active_support.deprecation = :log
+        config.active_support.test_order = :parallel
+        ActiveSupport::TestCase.respond_to?(:test_order=) && ActiveSupport::TestCase.test_order = :parallel
+        config.root = File.dirname(__FILE__)
+        config.log_level = :info
+        # Set a fake logger to avoid creating the log directory automatically
+        fake_logger = Logger.new(nil)
+        config.logger = fake_logger
+      end
+      @app.respond_to?(:secrets) && @app.secrets.secret_key_base = '3b7cd727ee24e8444053437c36cc66c4'
+
+      yield @app if block_given?
+      @app.initialize!
+    end
+  end
+end
+
+class ActiveSupport::TestCase
+  include TestHelpers::Generation
+end