Testing Action Mailer in Rails: Unit, Integration, and Preview

# config/environments/test.rb
Rails.application.configure do
  config.action_mailer.delivery_method = :test
  config.action_mailer.default_url_options = { host: 'example.com' }
end

# test/test_helper.rb (Minitest)
class ActiveSupport::TestCase
  setup { ActionMailer::Base.deliveries.clear }
end

# spec/rails_helper.rb (RSpec)
RSpec.configure do |config|
  config.before(:each) do
    ActionMailer::Base.deliveries.clear
  end
end

# app/mailers/user_mailer.rb
class UserMailer < ApplicationMailer
  def welcome_email(user)
    @user = user
    @login_url = login_url

    mail(
      to: user.email,
      subject: "Welcome to MyApp, #{user.first_name}!"
    )
  end

  def password_reset(user, token)
    @user = user
    @reset_url = password_reset_url(token: token)
    @expires_at = 2.hours.from_now

    mail(
      to: user.email,
      subject: "Reset your MyApp password"
    )
  end
end

# spec/mailers/user_mailer_spec.rb
RSpec.describe UserMailer do
  describe "#welcome_email" do
    let(:user) { create(:user, email: "alice@example.com", first_name: "Alice") }
    let(:mail) { UserMailer.welcome_email(user) }

    it "renders the correct recipient" do
      expect(mail.to).to eq(["alice@example.com"])
    end

    it "renders the subject with user's name" do
      expect(mail.subject).to eq("Welcome to MyApp, Alice!")
    end

    it "includes the login URL in the body" do
      expect(mail.body.encoded).to include("http://example.com")
    end

    it "addresses the user by name" do
      expect(mail.body.encoded).to include("Alice")
    end

    it "sends from the default from address" do
      expect(mail.from).to eq(["noreply@myapp.com"])
    end
  end

  describe "#password_reset" do
    let(:user) { create(:user) }
    let(:token) { "reset-token-abc123" }
    let(:mail) { UserMailer.password_reset(user, token) }

    it "includes the reset URL" do
      expect(mail.body.encoded).to include("reset-token-abc123")
    end

    it "mentions the expiry time" do
      expect(mail.body.encoded).to include("2 hours")
    end
  end
end

RSpec.describe "User registration" do
  it "sends a welcome email after signup" do
    expect {
      post "/users", params: {
        user: { email: "new@example.com", password: "password" }
      }
    }.to change(ActionMailer::Base.deliveries, :count).by(1)

    email = ActionMailer::Base.deliveries.last
    expect(email.to).to include("new@example.com")
    expect(email.subject).to include("Welcome")
  end
end

# With ActiveJob integration
RSpec.describe "Password reset" do
  it "enqueues a reset email" do
    user = create(:user)

    expect {
      post "/password/reset", params: { email: user.email }
    }.to have_enqueued_mail(UserMailer, :password_reset).with(user, anything)
  end
end

describe "#welcome_email" do
  let(:mail) { UserMailer.welcome_email(user) }

  it "sends a multipart email" do
    expect(mail.content_type).to start_with("multipart/alternative")
  end

  it "includes HTML part" do
    html_part = mail.html_part.body.decoded
    expect(html_part).to include("<h1>")
    expect(html_part).to include(user.first_name)
  end

  it "includes text part" do
    text_part = mail.text_part.body.decoded
    expect(text_part).not_to include("<h1>")  # no HTML tags in text
    expect(text_part).to include(user.first_name)
  end
end

# app/mailers/report_mailer.rb
class ReportMailer < ApplicationMailer
  def monthly_report(user, report_path)
    @user = user

    attachments["monthly-report.pdf"] = File.read(report_path)

    mail(to: user.email, subject: "Your Monthly Report")
  end
end

RSpec.describe ReportMailer do
  describe "#monthly_report" do
    let(:user) { create(:user) }
    let(:report_path) { Rails.root.join("spec/fixtures/sample-report.pdf") }
    let(:mail) { ReportMailer.monthly_report(user, report_path) }

    it "attaches the report" do
      expect(mail.attachments.count).to eq(1)
    end

    it "names the attachment correctly" do
      expect(mail.attachments.first.filename).to eq("monthly-report.pdf")
    end

    it "sets the correct MIME type" do
      expect(mail.attachments.first.mime_type).to eq("application/pdf")
    end
  end
end

# test/mailers/user_mailer_test.rb
class UserMailerTest < ActionMailer::TestCase
  test "welcome email" do
    user = users(:alice)
    mail = UserMailer.welcome_email(user)

    assert_equal ["alice@example.com"], mail.to
    assert_equal "Welcome to MyApp, Alice!", mail.subject
    assert_match "Alice", mail.body.encoded
    assert_match "login", mail.body.encoded
  end

  test "sends welcome email on create" do
    assert_emails 1 do
      post users_url, params: {
        user: { email: "new@test.com", password: "password" }
      }
    end
  end

  test "no email for invalid signup" do
    assert_no_emails do
      post users_url, params: {
        user: { email: "invalid", password: "" }
      }
    end
  end
end

# test/mailers/previews/user_mailer_preview.rb
class UserMailerPreview < ActionMailer::Preview
  def welcome_email
    user = User.first || FactoryBot.build(:user)
    UserMailer.welcome_email(user)
  end

  def password_reset
    user = User.first || FactoryBot.build(:user)
    UserMailer.password_reset(user, "preview-token-123")
  end

  def monthly_report
    user = User.first || FactoryBot.build(:user)
    report_path = Rails.root.join("spec/fixtures/sample-report.pdf")
    ReportMailer.monthly_report(user, report_path)
  end
end

describe "#welcome_email with locale" do
  it "sends email in user's locale" do
    user = create(:user, locale: "es")
    mail = UserMailer.welcome_email(user)

    expect(mail.subject).to eq("Bienvenido a MyApp, #{user.first_name}!")
    expect(mail.body.encoded).to include("Hola")
  end
end

def welcome_email(user)
  @user = user

  I18n.with_locale(user.locale) do
    mail(to: user.email, subject: t("mailer.welcome.subject", name: user.first_name))
  end
end