Rails No route matches missing required keys: [:id]

Question

I have this error with my a link in my views

HTML

<% if logged_in? %>
<%=link_to "View Your Cart", cart_path(@cart)%>
<% end %>

my routes

  resources :users
  resources :parts
  resources :carts
  resources :categories
  resources :line_items

I have this method here to assign a cart to a user

def set_cart
    @cart = Cart.find_by(id: session[:cart_id], user: session[:user_id])
  rescue ActiveRecord::RecordNotFound
    @cart = Cart.create
    session[:cart_id] = @cart.id
  end

Here is my sessions controller

def new
    @user = User.new
  end

  def create
    if params[:provider] ==  "facebook"
      user = User.from_omniauth(env["omniauth.auth"])
      session[:user_id] = user.id
      redirect_to root_path
    else
      @user = User.find_by(email: params[:user][:email])
      @user = User.new if @user.blank?
    if @user && @user.authenticate(params[:user][:password])
        session[:user_id] = @user.id
        @cart = Cart.create
        @user.cart = @cart.id 
        @user.save
          redirect_to @user
      else
        flash[:notice] = "Failed to login, please try again"
        render 'new'
      end
    end
  end

  def destroy
    session[:user_id] = nil
    redirect_to root_url
  end
end

And here is my Cart controller

class CartsController < ApplicationController
  before_action :set_cart, only: [:show, :edit, :update, :destroy]
  rescue_from ActiveRecord::RecordNotFound, with: :invalid_cart

  def show
    @cart = Cart.find(params[:id])
  end

  def edit
    @cart = Cart.new(cart_params)
  end

  def update
    @cart = Cart.find(params[:id])
      if @cart.update_attributes(cart_params)
        redirect_to @cart
      end
  end

  def destroy
    @cart.destroy if @cart.id == session[:cart_id]
    session[:cart_id] = nil
    respond_to do |format|
      format.html { redirect_to root_path }
      format.json { head :no_content }
  end
end

  private
  def cart_params
    params.require(:cart).permit(:user_id)
    end

   def invalid_cart
     logger.error "Attempt to access invalid cart #{params[:id]}"
     redirect_to root_path, notice: "Invalid cart"
   end
end

The following error "No route matches {:action=>"show", :controller=>"carts", :id=>nil} missing required keys: [:id]" rises up when a user signs into their account. What I want is for a user to have a "View Your Cart link" when they log in (inside the layout views) so they can view their cart anywhere. However this error raises once they log in. Any help with this one is appreciated and I'm happy to provide more info.

Answer 1

Try switching

Cart.find_by(id: session[:cart_id], user: session[:user_id])

with

Cart.find_by!(id: session[:cart_id], user: session[:user_id])

find_by returns nil if no record was found. find_by! throws ActiveRecord::RecordNotFound error.

Refer to ActiveRecord::FinderMethods for more info.