Adapt OrderContents to allow use with frontend cart service and updating :max_quantity

The cart service and it's surrounding logic is a bit messy, as it jumps through hoops to add extra logic for the :max_quantity attribute used with the "group buy" feature.

This is the last bit of code where an order's line items could be changed where we were not using OrderContents.

app/models/spree/order.rb

@@ -243,43 +243,11 @@ module Spree
       return_authorizations.any?(&:authorized?)
     end
 
-    # This is currently used when adding a variant to an order in the BackOffice.
-    # Spree::OrderContents#add is equivalent but slightly different from add_variant below.
+    # OrderContents should always be used when modifying an order's line items
     def contents
       @contents ||= Spree::OrderContents.new(self)
     end
 
-    # This is currently used when adding a variant to an order in the FrontOffice.
-    # This add_variant is equivalent but slightly different from Spree::OrderContents#add above.
-    # Spree::OrderContents#add is the more modern version in Spree history
-    #   but this add_variant has been customized for OFN FrontOffice.
-    def add_variant(variant, quantity = 1, max_quantity = nil)
-      line_items.reload
-      current_item = find_line_item_by_variant(variant)
-
-      # Notify bugsnag if we get line items with a quantity of zero
-      if quantity == 0
-        Bugsnag.notify(RuntimeError.new("Zero Quantity Line Item"),
-                       current_item: current_item.as_json,
-                       line_items: line_items.map(&:id),
-                       variant: variant.as_json)
-      end
-
-      if current_item
-        current_item.quantity = quantity
-        current_item.max_quantity = max_quantity
-        current_item.save
-      else
-        current_item = Spree::LineItem.new(quantity: quantity, max_quantity: max_quantity)
-        current_item.variant = variant
-        current_item.price = variant.price
-        line_items << current_item
-      end
-
-      reload
-      current_item
-    end
-
     # Associates the specified user with the order.
     def associate_user!(user)
       self.user = user

app/models/spree/order_contents.rb

@@ -26,6 +26,22 @@ module Spree
       line_item
     end
 
+    def update_or_create(variant, attributes)
+      line_item = find_line_item_by_variant(variant)
+
+      if line_item
+        line_item.update(attributes)
+      else
+        line_item = Spree::LineItem.new(attributes)
+        line_item.variant = variant
+        line_item.price = variant.price
+        order.line_items << line_item
+      end
+
+      order.reload
+      line_item
+    end
+
     def update_cart(params)
       if order.update_attributes(params)
         discard_empty_line_items

app/services/cart_service.rb

@@ -69,9 +69,9 @@ class CartService
   end
 
   def cart_add(variant, quantity, max_quantity)
-    final_quantity, final_max_quantity = final_quantities(variant, quantity, max_quantity)
-    if final_quantity > 0
-      @order.add_variant(variant, final_quantity, final_max_quantity)
+    attributes = final_quantities(variant, quantity, max_quantity)
+    if attributes[:quantity] > 0
+      @order.contents.update_or_create(variant, attributes)
     else
       @order.contents.remove(variant)
     end
@@ -84,7 +84,7 @@ class CartService
     final_quantity = [quantity, on_hand].min
     final_max_quantity = max_quantity # max_quantity is not capped
 
-    [final_quantity, final_max_quantity]
+    { quantity: final_quantity, max_quantity: final_max_quantity }
   end
 
   def overwrite_variants(variants)