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