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

  def products
    @products
  end

  def add_product(product)
    raise DuplicateProductIdError if product_by_id(product.id)

    @products.push(product)
  end

  def product_by_id(id)
    @products.find { |product| product.id == id }
  end
end