Railaでカート機能で保存できない

class CartsController < ApplicationController
  before_action :setup_cart_item!, only:[:create]

  def show
    @cart_items = current_cart.cart_item.includes([:product])
    @total = @cart_items.inject(0) { |sum, item| sum + item.sum_of_price }
  end

  def create
    if @cart_item.blank?
      @cart_item = current_cart.cart_items.build(product_id: params[:product_id])
    end

    @cart_item.quantity += params[:quantity].to_i
    @cart_item.save
    redirect_to root_path
  end
  
  private
    def setup_cart_item!
      @cart_item = current_cart.cart_items.find_by(product_id: params[:product_id])
    end
end

module CartsHelper
  
  def current_cart
    if current_user
      @current_cart = current_user.cart || current_user.create_cart
    else
      @current_cart = Cart.find(session[:cart_id]) || Cart.create
      session[:cart_id] ||= @current_cart.id
    end
  end
end

class Cart < ApplicationRecord
  has_many :cart_items, dependent: :destroy
end

class CartItem < ApplicationRecord
  belongs_to :product
  belongs_to :cart

  def sum_of_price
    product.price * quantity
  end
end

class Product < ApplicationRecord
  mount_uploaders :images, ProductUploader
  validates :images, presence: true
  validates :introduction, presence: true, length: { maximum: 500 }
  validates :name, presence: true, uniqueness: true, length: { maximum: 100 }
  validates :price, presence: true, numericality: { greater_than_or_equal_to: 1, less_than_or_equal_to: 10000 }
  default_scope -> { order(created_at: :desc) }
end

<%= provide(:title, "商品詳細") %>
<div class="sp-page">
  <div class="product_show">
    <div class="product_show-detail">
      <div class="product_show-name">
        <%= @product.name%>
      </div>
      <div class="product_show-price">
        <%= @product.price %>円/匹 (税込)
      </div>
      <div class="product_show-slide">
        <% @product.images.each do |img| %>
          <%= image_tag img.to_s, class:"product_show-img" %>
        <% end %>
      </div>

      <div class="product_show-introduction">
        <%= simple_format(@product.introduction) %>
      </div>

      <div class="product_show-cart">
        <div class="product_cart-price">
          <%= @product.price %>円/匹 (税込)
        </div>
        <%= form_with(model: @cart, local: true, url: add_item_url, method: :post) do |f| %>
          <%= f.label :数量 %>
          <%= f.select :quantity, [*1..50] %>
          <%= f.submit "カートへ追加", class:"product_show-btn"%>
        <% end %>
      </div>
    </div>
  </div>
  <div class="link-tab">
    <div class="link">
      <%= link_to "TOPへ戻る", '/' %>
    </div>
    <div class="link">
      <%= link_to "商品一覧へ戻る", '/products' %>
    </div>
  </div>
</div>

class ProductsController < ApplicationController
  before_action :admin_user, only:[:new, :create, :update, :destroy]

  def new
    @product = Product.new
  end

  def create
    @product = Product.new(product_params)
    if @product.save
      flash[:notice] = "商品を登録しました"
      redirect_to products_path
    else
      render 'new'
    end
  end

  def index
    @products = Product.all
  end

  def show
    @product = Product.find(params[:id])
    @cart = current_cart
  end
  
  def update

  end

  def destroy

  end

  private
    def product_params
      params.require(:product).permit(:name, :price, :introduction, {images: []})
    end
end

  def create
    if @cart_item.blank?
      @cart_item = current_cart.cart_items.build(product_id: 1)
    end

    @cart_item.quantity += params[:quantity].to_i
    @cart_item.save
    redirect_to root_path
  end