Round FlatPercentItemTotal calcs per-item before summing full quantity

app/models/spree/calculator/flat_percent_item_total_decorator.rb

@@ -0,0 +1,18 @@
+Spree::Calculator::FlatPercentItemTotal.class_eval do
+  # Spree's calculator sums all amounts, and then calculates a percentage on them.
+  # In the cart, we display line item individual amounts rounded, so to have consistent
+  # calculations we do the same internally. Here, we round adjustments at the individual
+  # item level first, then multiply by the item quantity.
+
+
+  def compute(object)
+    line_items_for(object).sum do |li|
+      unless li.price.present? && li.quantity.present?
+        raise ArgumentError, "object must respond to #price and #quantity"
+      end
+
+      value = (li.price * BigDecimal(self.preferred_flat_percent.to_s) / 100.0).round(2)
+      value * li.quantity
+    end
+  end
+end

spec/features/consumer/shopping/cart_spec.rb

@@ -13,16 +13,35 @@ feature "full-page cart", js: true do
     let!(:order_cycle) { create(:simple_order_cycle, suppliers: [supplier], distributors: [distributor], coordinator: create(:distributor_enterprise), variants: [product_tax.variants.first, product_fee.variants.first]) }
     let(:enterprise_fee) { create(:enterprise_fee, amount: 11.00, tax_category: product_tax.tax_category) }
     let(:product_tax) { create(:taxed_product, supplier: supplier, zone: zone, price: 110.00, tax_rate_amount: 0.1) }
-    let(:product_fee) { create(:simple_product, supplier: supplier, price: 0.86) }
+    let(:product_fee) { create(:simple_product, supplier: supplier, price: 0.86, on_hand: 100) }
     let(:order) { create(:order, order_cycle: order_cycle, distributor: distributor) }
 
     before do
-      add_enterprise_fee enterprise_fee
       set_order order
     end
 
+    describe "fees" do
+      let(:percentage_fee) { create(:enterprise_fee, calculator: Spree::Calculator::FlatPercentItemTotal.new(preferred_flat_percent: 20)) }
+
+      before do
+        add_enterprise_fee percentage_fee
+        add_product_to_cart order, product_fee, quantity: 8
+        visit spree.cart_path
+      end
+
+      it "rounds fee calculations correctly" do
+        # $0.86 + 20% = $1.032
+        # Fractional cents should be immediately rounded down and not carried through
+        expect(page).to have_selector '.cart-item-price',         text: '$1.03'
+        expect(page).to have_selector '.cart-item-total',         text: '$8.24'
+        expect(page).to have_selector '.order-total.item-total',  text: '$8.24'
+        expect(page).to have_selector '.order-total.grand-total', text: '$8.24'
+      end
+    end
+
     describe "tax" do
       before do
+        add_enterprise_fee enterprise_fee
         add_product_to_cart order, product_tax
         visit spree.cart_path
       end

spec/models/spree/calculator/flat_percent_item_total_spec.rb

@@ -0,0 +1,15 @@
+module Spree
+  describe Calculator::FlatPercentItemTotal do
+    let(:calculator) { Calculator::FlatPercentItemTotal.new preferred_flat_percent: 20 }
+
+    it "calculates for a simple line item" do
+      line_item = LineItem.new price: 50, quantity: 2
+      expect(calculator.compute(line_item)).to eq 20
+    end
+
+    it "rounds fractional cents before summing" do
+      line_item = LineItem.new price: 0.86, quantity: 8
+      expect(calculator.compute(line_item)).to eq 1.36
+    end
+  end
+end

spec/support/request/shop_workflow.rb

@@ -16,9 +16,9 @@ module ShopWorkflow
     ApplicationController.any_instance.stub(:session).and_return({order_id: order.id, access_token: order.token})
   end
 
-  def add_product_to_cart(order, product)
+  def add_product_to_cart(order, product, quantity: 1)
     populator = Spree::OrderPopulator.new(order, order.currency)
-    populator.populate(variants: {product.variants.first.id => 1})
+    populator.populate(variants: {product.variants.first.id => quantity})
 
     # Recalculate fee totals
     order.update_distribution_charge!