domain_constraint.rb

class DomainConstraint
  def initialize(domain)
    @domains = [domain].flatten
  end

  def matches?(request)
    request.subdomain.present? ? domain_to_match = request.subdomain + "." + request.domain : domain_to_match = request.domain
    @domains.include? domain_to_match
  end
end

integration_spec.rb

require 'spec_helper'

describe "Something", js: true do
  it "is a fake spec" do
    # this won't work in CI/test environments
    # 'another.localhost:3001' is not mapped in /etc/hosts
    visit foo_path(subdomain: 'another')
  end
end

routes.rb

MyApp::Application.routes.draw do
  constraints DomainConstraint.new(['subdomain.domain.com', 'another.domain.com']) do
    resources :foo
  end
end

url_helper.rb

module UrlHelper
  def with_subdomain(subdomain)
    subdomain = (subdomain || "")
    subdomain += "." unless subdomain.empty?
    [subdomain, request.domain].join
  end

  # allow link_to :subdomain => ""
  def url_for(options = nil)
    if options.kind_of?(Hash) && options.has_key?(:subdomain)
      options[:host] = with_subdomain(options.delete(:subdomain))
    end
    super
  end
end