如果是POST，为什么需要在response_with中指定：location => some_url（@object）

Question

I am implementing a RESTful API using RoR. Here my UsersController :

class ApplicationController < ActionController::API
    [...]
end

module Api
  module V1
    class UsersController < ApplicationController

      respond_to :json

      def create
        @user = User.new(user_params)
        @user.save
        respond_with @user, :location => api_v1_user_url(@user)
      end

      def show
        @user = User.find(params[:id])
        respond_with @user
      end

    end
  end
end

If I omit the @user param from :location => api_v1_user_url(@user) in UsersController#create ie I have it like so:

respond_with @user, :location => api_v1_user_url

I get the following error:

ActionController::UrlGenerationError in Api::V1::UsersController#create
No route matches {:action=>"show", :controller=>"api/v1/users", :format=>"json"} missing required keys: [:id]

I just added the @user param to api_v1_user_url on a hunch after I saw this error and it works.

My question is what is going on here? I do not understand why I need to pass @user when most of the examples I've seen do not involve passing the object @user to the :location attribute.

The output of rake routes is a follows:

$ rake routes
Ignoring bcrypt-3.1.7 because its extensions are not built.  Try: gem pristine bcrypt-3.1.7
          Prefix Verb   URI Pattern                      Controller#Action
    api_v1_users GET    /api/v1/users(.:format)          api/v1/users#index {:format=>"json"}
                 POST   /api/v1/users(.:format)          api/v1/users#create {:format=>"json"}
 new_api_v1_user GET    /api/v1/users/new(.:format)      api/v1/users#new {:format=>"json"}
edit_api_v1_user GET    /api/v1/users/:id/edit(.:format) api/v1/users#edit {:format=>"json"}
     api_v1_user GET    /api/v1/users/:id(.:format)      api/v1/users#show {:format=>"json"}
                 PATCH  /api/v1/users/:id(.:format)      api/v1/users#update {:format=>"json"}
                 PUT    /api/v1/users/:id(.:format)      api/v1/users#update {:format=>"json"}
                 DELETE /api/v1/users/:id(.:format)      api/v1/users#destroy {:format=>"json"}
    static_index GET    /static/index(.:format)          static#index
            root GET    /                                static#index

Another place I've seen this (just to name another one):

• Embracing REST with mind, body and soul

Answer 1

The reason being is that api_v1_user_url or api_v1_user_path are for a single instance and you need to pass the id of that user it should look for and render. Hence why you got the error you go.

In the examples you probably saw api_v1_users_url or api_v1_users_path which renders all the users. Notice the users_ vs user_