Prevent users from removing the final line item of an order, suggest cancelling instead

app/controllers/spree/orders_controller_decorator.rb

@@ -8,6 +8,7 @@ Spree::OrdersController.class_eval do
   prepend_before_filter :require_order_cycle, only: :edit
   prepend_before_filter :require_distributor_chosen, only: :edit
   before_filter :check_hub_ready_for_checkout, only: :edit
+  before_filter :check_at_least_one_line_item, only: :update
 
   include OrderCyclesHelper
   layout 'darkswarm'
@@ -223,4 +224,17 @@ Spree::OrdersController.class_eval do
     return order if order.andand.changes_allowed? && can?(:update, order)
     current_order
   end
+
+  def check_at_least_one_line_item
+    order = order_to_update
+    return unless order.complete?
+
+    items = params[:order][:line_items_attributes]
+    .andand.select{ |k,attrs| attrs["quantity"].to_i > 0 }
+
+    if items.empty?
+      flash[:error] = I18n.t(:orders_cannot_remove_the_final_item)
+      redirect_to order_path(order)
+    end
+  end
 end

config/locales/en.yml

@@ -1060,6 +1060,7 @@ See the %{link} to find out more about %{sitename}'s features and to start using
   orders_show_confirmed: Confirmed
   orders_your_order_has_been_cancelled: "Your order has been cancelled"
   orders_could_not_cancel: "Sorry, the order could not be cancelled"
+  orders_cannot_remove_the_final_item: "Cannot remove the final item from an order, please cancel the order instead."
   orders_bought_items_notice:
     one: An additional item is already confirmed for this order cycle
     other: "%{count} additional items already confirmed for this order cycle"

spec/controllers/spree/orders_controller_spec.rb

@@ -191,6 +191,40 @@ describe Spree::OrdersController do
     end
   end
 
+  describe "removing items from a completed order" do
+    let(:order) { create(:completed_order_with_totals) }
+    let!(:line_item) { order.reload.line_items.first }
+    let(:params) { { order: {} } }
+
+    before { allow(subject).to receive(:order_to_update) { order } }
+
+    context "when more than one item remains" do
+      before do
+        params[:order][:line_items_attributes] = { "0" => {quantity: "1", id: line_item.id} }
+      end
+
+      it "does not remove the item, flash suggests cancellation" do
+        spree_post :update, params
+        expect(flash[:error]).to be nil
+        expect(response).to redirect_to spree.order_path(order)
+        expect(order.reload.line_items.count).to eq 1
+      end
+    end
+
+    context "when only one item remains" do
+      before do
+        params[:order][:line_items_attributes] = { "0" => {quantity: "0", id: line_item.id} }
+      end
+
+      it "does not remove the item, flash suggests cancellation" do
+        spree_post :update, params
+        expect(flash[:error]).to eq I18n.t(:orders_cannot_remove_the_final_item)
+        expect(response).to redirect_to spree.order_path(order)
+        expect(order.reload.line_items.count).to eq 1
+      end
+    end
+  end
+
   describe "#order_to_update" do
     let!(:current_order) { double(:current_order) }
     let(:params) { { } }