使用嵌套路由Rails 3而不实际允许访问父资源

resources :posts do
  resources :comments
end

post_comments GET    /posts/:post_id/comments(.:format)          comments#index
               POST   /posts/:post_id/comments(.:format)          comments#create
new_post_comment GET    /posts/:post_id/comments/new(.:format)      comments#new
edit_post_comment GET    /posts/:post_id/comments/:id/edit(.:format) comments#edit
post_comment GET    /posts/:post_id/comments/:id(.:format)      comments#show
               PATCH  /posts/:post_id/comments/:id(.:format)      comments#update
               PUT    /posts/:post_id/comments/:id(.:format)      comments#update
               DELETE /posts/:post_id/comments/:id(.:format)      comments#destroy

class CommentsController< ApplicationController
  before_action :set_post
  before_action :check_permission, only: [:index, :create, :new, :edit, :update, :destroy]

  # ...

  private

  def set_post
    @post = Post.find(params[:post_id])
  end

  def check_permission
    unless @post.can_access?(current_user)
      redirect_to root_path, alert: "You do not have permission to access this resource."
    end
  end
end