constraints.rb

# config/initializers/constraints.rb
class Home
  def matches?(req)
    req.params[:id] #=> nil
    # .....
  end
end

home_controller.rb

# app/controllers/home_controller.rb
class HomeController < ApplicationController
  def show
    render text: params[:id]
  end
end

home_routing_spec.rb

# spec/routing/home_routing_spec.rb
require 'spec_helper'

describe 'routes to the home controller' do
  it 'routes /home/1 to home#show' do
    get('/home/1').should route_to controller: 'home', action: 'show', id: '1'
  end
end

routes.rb

# config/routes.rb
Spike::Application.routes.draw do
  get "home/:id" => 'home#show', constraints: Home.new
end

• rake spec すると constraints .rb の req.params[:id] #=> nil になる
• rails server して http://localhost:3000/home/1 すると req.params[:id] #=> "1" になる

gem 'rails', '3.2.21'

rails/rails@5603050

4.2.0 は OK