假设有这样的电子邮件：

class UserMailer < ApplicationMailer
  def welcome_email(user)
    @user = user
    mail(to: @user.email, subject: 'Welcome')
  end
end

然后这样发邮件：

class User < ApplicationRecord
  def self.register(email, password)
    user = create(email: email, password: password)
    UserMailer.welcome_email(user).deliver_later
  end
end

使用 deliver_later 方法需要在 application.rb 里加上：

config.active_job.queue_adapter = :sidekiq # 可以换成其它的 adapter

如果邮件模板中有外链，还需要在 environments/test.rb 里加上：

config.action_mailer.default_url_options = { host: 'test.mysite.com' }

在 user_spec.rb 里：

require 'rails_helper'

RSpec.describe User, type: :model do

  describe '.register' do
    include ActiveJob::TestHelper

    it 'should be created mail job' do
      ActiveJob::Base.queue_adapter = :test
      expect {
        User.register('[email protected]', '123123')
      }.to have_enqueued_job.on_queue('mailers')
    end

    it 'should be sent welcome mail' do
      expect {
        perform_enqueued_jobs do
          User.register('[email protected]', '123123')
        end
      }.to change { ActionMailer::Base.deliveries.size }.by(1)
    end

    it 'should be sent to the right user' do
      perform_enqueued_jobs do
        User.register('[email protected]', '123123')
      end

      mail = ActionMailer::Base.deliveries.last
      expect(mail.to[0]).to eq "[email protected]"
    end
  end
end