reset `ActionMailer::Base.deliveries` in `ActionDispatch::IntegrationTest`.

Whenever you are sending emails in integration tests using the `:test`
delivery method you need to make sure that
`ActionMailer::Base.deliveries` is reset after every test. This piece of
boilerplate code is present in all my applications that send
emails. Let's have `ActionDispatch::IntegrationTest` reset the
deliveries automatically.

actionmailer/CHANGELOG.md

+*   Reset `ActionMailer::Base.deliveries` after every test in
+    `ActionDispatch::IntegrationTest`.
+
+    *Yves Senn*
+
+
 ## Rails 5.0.0.beta2 (February 01, 2016) ##
 
 *   No changes.

actionmailer/lib/action_mailer/railtie.rb

@@ -51,6 +51,8 @@ class Railtie < Rails::Railtie # :nodoc:
             get '/rails/mailers/*path'   => "rails/mailers#preview"
           end
         end
+
+        ActionDispatch::IntegrationTest.send :include, ActionMailer::TestCase::ClearTestDeliviers
       end
     end
 

actionmailer/lib/action_mailer/test_case.rb

@@ -11,6 +11,21 @@ def initialize(name)
   end
 
   class TestCase < ActiveSupport::TestCase
+    module ClearTestDeliviers
+      extend ActiveSupport::Concern
+
+      included do
+        teardown :clear_test_deliviers
+      end
+
+      private
+      def clear_test_deliviers
+        if ActionMailer::Base.delivery_method == :test
+          ActionMailer::Base.deliveries.clear
+        end
+      end
+    end
+
     module Behavior
       extend ActiveSupport::Concern
 
@@ -66,7 +81,6 @@ def initialize_test_deliveries # :nodoc:
         def restore_test_deliveries # :nodoc:
           restore_delivery_method
           ActionMailer::Base.perform_deliveries = @old_perform_deliveries
-          ActionMailer::Base.deliveries.clear
         end
 
         def set_delivery_method(method) # :nodoc:
@@ -100,5 +114,6 @@ def read_fixture(action)
     end
 
     include Behavior
+    include ClearTestDeliviers
   end
 end

guides/source/testing.md

@@ -1191,9 +1191,9 @@ testing) but instead it will be appended to an array
 (`ActionMailer::Base.deliveries`).
 
 NOTE: The `ActionMailer::Base.deliveries` array is only reset automatically in
-`ActionMailer::TestCase` tests. If you want to have a clean slate outside Action
-Mailer tests, you can reset it manually with:
-`ActionMailer::Base.deliveries.clear`
+`ActionMailer::TestCase` and `ActionDispatch::IntegrationTest` tests.
+If you want to have a clean slate outside these test cases, you can reset it
+manually with: `ActionMailer::Base.deliveries.clear`
 
 ### Functional Testing
 

railties/test/application/integration_test_case_test.rb

+require 'isolation/abstract_unit'
+
+module ApplicationTests
+  class IntegrationTestCaseTest < ActiveSupport::TestCase
+    include ActiveSupport::Testing::Isolation
+
+    setup do
+      build_app
+      boot_rails
+    end
+
+    teardown do
+      teardown_app
+    end
+
+    test "resets Action Mailer test deliveries" do
+      script('generate mailer BaseMailer welcome')
+
+      app_file 'test/integration/mailer_integration_test.rb', <<-RUBY
+        require 'test_helper'
+
+        class MailerIntegrationTest < ActionDispatch::IntegrationTest
+          setup do
+            @old_delivery_method = ActionMailer::Base.delivery_method
+            ActionMailer::Base.delivery_method = :test
+          end
+
+          teardown do
+            ActionMailer::Base.delivery_method = @old_delivery_method
+          end
+
+          2.times do |i|
+            define_method "test_resets_deliveries_\#{i}" do
+              BaseMailer.welcome.deliver_now
+              assert_equal 1, ActionMailer::Base.deliveries.count
+            end
+          end
+        end
+      RUBY
+
+      output = Dir.chdir(app_path) { `bin/rails test 2>&1` }
+      assert_equal 0, $?.to_i, output
+      assert_match /0 failures, 0 errors/, output
+    end
+  end
+end