"should get show" controller test failing for Nested resource

I have a User model and a Shoppingcart model where

# user.rb
class User < ActiveRecord::Base
    has_one :shoppingcart
end

and

# shopppingcart.rb
class Shoppingcart < ActiveRecord::Base
    belongs_to :user
end

and I have this in routes.rb:

resources :users do
    resource :shoppingcart
end

and in Shoppingcarts_controller.rb I have

class ShoppingcartsController < ApplicationController
    def show
        @user = User.find(current_user)
        @shoppingcart = @user.build_shoppingcart
    end
end

and within shoppingcarts_controller_test.rb:

require 'test_helper'
class ShoppingcartsControllerTest < ActionController::TestCase
    def setup
        @user = users(:michael)
    end
    test "should get show" do
        puts(@user.name) # => Michael Example
        puts(@user.id) # => 762146111
        get :show # error line
        assert_response :success
    end
end

But when ever I run the test I get ActiveRecord::RecordNotFound: Couldn't find User with 'id'=. When I comment out the error line the error goes away and everything works. As for the non-test code, everything works. I can reach users/1/shoppingcart with no issue, so the problem must be coming from the test itself. How do I make a test for an action to a nested resource?

SessionsHelper.rb:

module SessionsHelper

    # Returns the current logged-in user (if any).
    def current_user
        if (user_id = session[:user_id])
            @current_user ||= User.find_by(id: user_id)
        elsif (user_id = cookies.signed[:user_id])
            user = User.find_by(id: user_id)
            if user && user.authenticated?(:remember, cookies[:remember_token])
                log_in user
                @current_user = user
            end
        end
    end
end 

find method requires primary key of the particular user from Users table, not the user itself. Primary key is id, so you need to change this line in your controller.

@user = User.find(params[:user_id])