Ruby on Rails: wrong number of arguments (given 0, expected 1) - ruby-on-rails

Question

I have an issue with my add_item method and have trouble to understand why.
Here is my carts_controller.rb
class CartsController < ApplicationController
def index
#cart_items = CartItem.all
end
def add_item
#cart_item = CartItem.new
produit_id = params[:produit_id]
#cart_item = CartItem.find_or_create_by(params[:produit][:produit_id])
#cart_item.save
binding.pry
end
end
Here is produits/index.html.erb (where the issue comes from)
<div id="produits-column-container">
<% if #produits %>
<% #produits.in_groups_of(4, false).each do |g| %>
<% g.each do |produit| %>
<div id="produits-row-container">
<div id="fiche-produit-container">
<div id="produit-img">
<%= image_tag produit.photo %>
</div>
<div id="produit-nom">
<%= produit.nom %>
</div>
<div id="produit-prix">
<%= number_to_currency(produit.prix, unit: '€', format: "%n%u") %>
</div>
<div id="produit-au-panier">
<%= image_tag('icon/icon-panier') %>
<%= link_to 'Ajouter au panier', carts_add_item_path, method: :post %>
</div>
</div>
</div>
<% end %>
<% end %>
<% end %>
</div>
The error i'm given is :
ArgumentError in CartsController#add_item
wrong number of arguments (given 0, expected 1)
in def add_item(produit_id)
add_item(produit_id) is related to carts_add_item_path
I also give you the routes :
Rails.application.routes.draw do
match "/mon-panier" => 'carts#index', via: :get
post 'carts/add_item' => 'carts#add_item'
resources :categories do
resources :produits
end
resources :order_abonnements, only: [:create, :update, :delete]
get 'livraisons_type/index'
match "/recapitulatif" => 'recapitulatif#index', via: :get
match "/confirmation-carte-cadeau" => 'recapitulatif#confirmation', via: :get
match "/livraison-carte-cadeau" => 'livraison_carte#index', via: :get
match '/activation-carte' => 'code_carte_cadeau#index', via: :get
match "/offrir-une-box-bretonne" => 'cadeau#index', via: :get
resources :order_items, only: [:create, :update, :destroy]
match "/nos-box" => 'nos_box#index', via: :get
get 'categories/index'
devise_for :admin_users, ActiveAdmin::Devise.config
ActiveAdmin.routes(self)
match '/informations-penn-ar-box' => 'informations_penn_ar_box#index', via: :get
match '/livraison-box-bretonne' => 'livraison_box_bretonne#index', via: :get
match '/abonnements' => 'abonnements#index', via: :get
devise_for :users, path: '', path_names: { sign_in: 'connexion', sign_out: 'déconnexion'}
resources :users do
delete 'déconnexion' => 'devise/sessions#destroy'
end
match '/mon-marche-breton' => 'marche_breton#index', via: :get
root 'home#home'
end
And the logs :
Started POST "/carts/add_item" for ::1 at 2017-05-30 09:48:52 +0200
Processing by CartsController#add_item as HTML
Parameters: {"authenticity_token"=>"QrToQUHVxjuV5cUvZYHd7tj457htfZohOkmsvNDnKv79P413xjsSfR/8RVXtdIU7/wcmhcxjkU85N13CqJkG2w=="}
Cart Load (0.3ms) SELECT `carts`.* FROM `carts` WHERE `carts`.`id` = 1 LIMIT 1
Completed 500 Internal Server Error in 27ms (ActiveRecord: 14.9ms)
ArgumentError (wrong number of arguments (given 0, expected 1)):
app/controllers/carts_controller.rb:6:in `add_item'

Answer 1

You need to pass produit_id as a parameter.
So change this line as so...
<%= link_to 'Ajouter au panier',
carts_add_item_path(produit_id: produit.id), method: :post %>
And change your controller method as so...
def add_item
produit_id = params[:produit_id]
...
And change the find_or_create to
#cart_item = CartItem.find_or_create_by(produit_id: produit_id)
This does mean that you can only have one CartItem in your entire application that points to a product... strange design.

Answer 2

There are a lot of things that are wrong with this code, but the simplest thing you could do in order to suppress the error is to change the method signature to:
def add_item
instead of
def add_item(produit_id)
P.S. The controller is not RESTful. It's called CartsController, but in your index action you list the CartItems, not the Carts. The correct name should be CartItemsController. If you rename the controller to CartItemsController, than the index action can stay the same, but the add_item action should better be renamed to simply create. Thus, in your routes you can have:
resources :cart_items, only: [:index, :create]
Also, I am not exactly sure, what is going on in the add_item method - at first you assign a new CartItem to #cart_item, but then you override this assignment with find_or_create_by.... Also, the call to save at the end of the method is redundant, as create will save the record and otherwise no modifications have been made between the find_or_create_by line and the save line.