Rails测试疑问：ActiveMailer发送后delivery_method.settings被清空？

我需要验证UserMailer是否按照租户指定的SMTP服务器发送邮件，在UserMailer中通过before_action钩子动态设置SMTP配置：

class UserMailer < ApplicationMailer
  before_action :set_user
  before_action :set_smtp_settings

  # ... 其他代码 ...

  def set_smtp_settings
    tenant_email_provider = @user.tenant.email_providers.where(provider_type: 'smtp').take
    tenant_smtp_settings = (tenant_email_provider&.settings || {}).with_indifferent_access
    
    current_settings = (message.delivery_method.settings || {} ).with_indifferent_access
    new_settings = current_settings.merge(tenant_smtp_settings)
    
    raise "No server address to send to" if new_settings[:address].nil?
    
    message.delivery_method.settings = new_settings
  end
end

单元测试代码如下：

test 'that the settings are according to a mailer' do
  expected_settings = email_provider.settings.with_indifferent_access

  message = @mailer.account_activation_confirm
  sent_message = message.deliver_now

  actual_settings = sent_message.delivery_method.settings.with_indifferent_access

  intersecting_keys = expected_settings.keys.map(&:to_sym) | actual_settings.keys
  assert intersecting_keys.include?(:address)

  intersecting_keys.each do |setting|
    expected = expected_settings[setting].to_s
    actual = actual_settings[setting].to_s
    assert_equal(expected, actual, "smtp value of setting '#{setting}' do not match (#{expected} != #{actual})")
  end
end

测试失败，报错信息：

Failure:
DeliverSettingsTest#test_that_the_settings_are_according_to_a_mailer [/opt/xxx-web/test/mailers/deliver_settings_test.rb:25]:
smtp value of setting 'address' do not match (mail.hello.com != ).
--- expected
+++ actual
@@ -1 +1,3 @@
-"mail.hello.com"
+# encoding: US-ASCII
+#    valid: true
+""

通过调试发现：

• 在set_smtp_settings钩子中，message.delivery_method.settings已正确填充租户的SMTP配置：

  (ruby) message.delivery_method.settings
  {"address"=>"mail.hello.com",
   "port"=>25,
   "domain"=>"localhost.localdomain",
   "user_name"=>"prius",
   "password"=>"bunch_tr1k3s!",
   "authentication"=>nil,
   "enable_starttls"=>nil,
   "enable_starttls_auto"=>true,
   "openssl_verify_mode"=>nil,
   "ssl"=>nil,
   "tls"=>nil,
   "open_timeout"=>5,
   "read_timeout"=>5}
  

• 但在测试代码中，sent_message.delivery_method.settings是空哈希：

  (ruby) actual_settings
  {}
  

疑问：是否是ActiveMailer库或测试桩在邮件发送后清空了delivery_method.settings的内容？

确实是测试环境下ActiveMailer的默认行为导致的：

1. 测试环境默认使用:test投递方式：测试模式中，ActiveMailer不会真正通过SMTP发送邮件，而是用ActionMailer::DeliveryMethods::Test这个桩实现，仅把邮件存入内存的ActionMailer::Base.deliveries数组，不会保留原SMTP投递方式的配置。
2. 发送后返回的sent_message已切换为测试桩：调用deliver_now后，邮件对象的投递方式会被替换为测试用的实现，你之前设置的SMTP配置不会保留在返回的sent_message中。

可以通过两种方式调整测试逻辑：

方案1：发送前直接验证邮件对象的配置 #

无需等待发送，直接在调用deliver_now前检查message的投递配置：

test 'that the settings are according to a mailer' do
  expected_settings = email_provider.settings.with_indifferent_access

  message = @mailer.account_activation_confirm
  # 发送前验证配置
  actual_settings = message.delivery_method.settings.with_indifferent_access

  intersecting_keys = expected_settings.keys.map(&:to_sym) | actual_settings.keys
  assert intersecting_keys.include?(:address)

  intersecting_keys.each do |setting|
    expected = expected_settings[setting].to_s
    actual = actual_settings[setting].to_s
    assert_equal(expected, actual, "smtp value of setting '#{setting}' do not match (#{expected} != #{actual})")
  end

  # 可选：若需确保邮件能正常发送，保留发送操作
  message.deliver_now
end

方案2：用Mock验证SMTP配置被正确应用 #

如果要验证配置确实被应用到发送流程，可借助Minitest的Mock功能拦截SMTP初始化：

test 'that the settings are according to a mailer' do
  expected_settings = email_provider.settings.with_indifferent_access
  # 预期的完整配置（含默认值）
  expected_full_settings = message.delivery_method.settings.with_indifferent_access.merge(expected_settings)

  # Mock SMTP投递方式，验证初始化时使用了正确配置
  smtp_mock = Minitest::Mock.new
  smtp_mock.expect(:deliver!, nil, [instance_of(Mail::Message)])
  smtp_mock.expect(:settings=, nil, [expected_full_settings])

  # 替换邮件对象的投递方式为mock
  message = @mailer.account_activation_confirm
  message.delivery_method(smtp_mock)

  message.deliver_now

  # 验证mock的预期调用是否执行
  smtp_mock.verify
end

内容的提问来源于stack exchange，提问作者Julien Lamarche