ruby on rails - Why do I need to specify :location=>some_url(@object) in respond_with in the case of POST -

i implementing restful api using ror. here 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 omit @user param :location => api_v1_user_url(@user) in userscontroller#create i.e. have so:

respond_with @user, :location => api_v1_user_url 

i 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 added @user param api_v1_user_url on hunch after saw error , works.

my question going on here? not understand why need pass @user when of examples i've seen not involve passing object @user :location attribute.

the output of rake routes follows:

$ rake routes ignoring bcrypt-3.1.7 because extensions not built.  try: gem pristine bcrypt-3.1.7           prefix verb   uri pattern                      controller#action     api_v1_users    /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    /api/v1/users/new(.:format)      api/v1/users#new {:format=>"json"} edit_api_v1_user    /api/v1/users/:id/edit(.:format) api/v1/users#edit {:format=>"json"}      api_v1_user    /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    /static/index(.:format)          static#index             root    /                                static#index 

another place i've seen (just name one):

• embracing rest mind, body , soul

the reason being api_v1_user_url or api_v1_user_path single instance , need pass id of user should , render. hence why got error go.

in examples saw api_v1_users_url or api_v1_users_path renders users. notice users_ vs user_