Add Shop class to keep track of available products

shop.rb

@@ -0,0 +1,22 @@
+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

test/shop_test.rb

@@ -0,0 +1,25 @@
+require "minitest/autorun"
+
+require "shop.rb"
+require "product.rb"
+
+class ShopTest < Minitest::Test
+  def setup
+    @shop = Shop.new
+  end
+
+  def test_adding_a_product
+    pico = Product.new(id: 'PICO2', price: 4.00)
+    @shop.add_product(pico)
+
+    assert_equal [pico], @shop.products
+  end
+
+  def test_duplicate_product_id_raises_an_error
+    @shop.add_product(Product.new(id: 'PICO2'))
+
+    assert_raises Shop::DuplicateProductIdError do
+     @shop.add_product(Product.new(id: 'PICO2'))
+    end
+  end
+end