rails/actionmailer/test/asset_host_test.rb

require "abstract_unit"
require "action_controller"

class AssetHostMailer < ActionMailer::Base
  def email_with_asset
    mail to: "test@localhost",
      subject: "testing email containing asset path while asset_host is set",
      from: "tester@example.com"
  end
end

class AssetHostTest < ActionMailer::TestCase
  def setup
    AssetHostMailer.configure do |c|
      c.asset_host = "http://www.example.com"
    end
  end

  def teardown
    restore_delivery_method
  end

  def test_asset_host_as_string
    mail = AssetHostMailer.email_with_asset
    assert_dom_equal '<img alt="Somelogo" src="http://www.example.com/images/somelogo.png" />', mail.body.to_s.strip
  end

  def test_asset_host_as_one_argument_proc
    AssetHostMailer.config.asset_host = Proc.new { |source|
      if source.starts_with?("/images")
        "http://images.example.com"
      end
    }
    mail = AssetHostMailer.email_with_asset
    assert_dom_equal '<img alt="Somelogo" src="http://images.example.com/images/somelogo.png" />', mail.body.to_s.strip
  end
end