class Shop
  class DuplicateProductIdError < StandardError; end
  def initialize
    @products = []
  end

  def products
    @products
  end

  def add_product(product)
    raise DuplicateProductIdError if duplicate_id?(product.id)

    @products.push(product)
  end

  private

  def duplicate_id?(id)
    @products.map(&:id).include?(id)
  end
end