[Solved] How do I prevent the addition of the same product to the cart [closed]

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Add uniqueness validation on Cart.product_id, scoping by Cart.id:

class Cart < ApplicationRecord
  validates :product_id, uniqueness: {scope: :id}
end

But beware of race conditions.

UPDATE: If no actual Cart model add validation to LineItem:

class LineItem < ApplicationRecord
  validates :product_id, uniqueness: {scope: :order_id}
end

UPDATE 2: refactor add method with find_or_initialize_by:

def add
  @cart.save if @cart.new_record?
  session[:cart_id] = @cart.id
  product = Product.find(params[:id])
  line_item = LineItem.find_or_initialize_by(order:   @cart,
                                             product: product)
  line_item.price = product.price
  line_item.save!
  @cart.recalculate_price!
  flash[:notice] = "Item added to cart!"
  redirect_to '/cart'
end

UPDATE 3: Checking for product existence:

def add
  @cart.save if @cart.new_record?
  session[:cart_id] = @cart.id
  product = Product.find(params[:id])
  line_item = LineItem.find_by(order: @cart, product: product)
  if line_item
   notice = "ERROR: Product already in the cart"
  else   
   LineItem.create!(order:   @cart,
                    product: product,
                    price:   product.price)
   @cart.recalculate_price!
   notice = "Item added to cart!"
  end
  flash[:notice] = notice
  redirect_to '/cart'
end

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solved How do I prevent the addition of the same product to the cart [closed]