update the logic to send invoice

When the invoices feature is enabled, for every order, we check if
1. a new invoice must be generated
2. or, the latest invoice needs to be updated

the invoice rendrer input depends on the invoices flag.
if the feature is enabled, the input is supposed to be an invoice presenter

app/mailers/spree/order_mailer.rb

@@ -48,7 +48,14 @@ module Spree
 
     def invoice_email(order_or_order_id)
       @order = find_order(order_or_order_id)
-      pdf = InvoiceRenderer.new.render_to_string(@order)
+      renderer_data = if OpenFoodNetwork::FeatureToggle.enabled?(:invoices)
+                        OrderInvoiceGenerator.new(@order).generate_or_update_latest_invoice
+                        @order.invoices.first.presenter
+                      else
+                        @order
+                      end
+
+      pdf = InvoiceRenderer.new.render_to_string(renderer_data)
 
       attach_file("invoice-#{@order.number}.pdf", pdf)
       I18n.with_locale valid_locale(@order.user) do

spec/mailers/order_mailer_spec.rb

@@ -216,4 +216,41 @@ describe Spree::OrderMailer do
       end
     end
   end
+
+  describe "#invoice_email" do
+    subject(:email) { described_class.invoice_email(order) }
+    let(:order) { create(:completed_order_with_totals) }
+    let!(:invoice_data_generator){ InvoiceDataGenerator.new(order) }
+    let!(:invoice){
+      create(:invoice, order:,
+                       data: invoice_data_generator.serialize_for_invoice)
+    }
+
+    let(:generator){ double(:generator) }
+    let(:renderer){ double(:renderer) }
+    before do
+      allow(OrderInvoiceGenerator).to receive(:new).with(order).and_return(generator)
+      allow(InvoiceRenderer).to receive(:new).and_return(renderer)
+    end
+    context "When invoices feature is not enabled" do
+      it "should call the invoice render with order as argument" do
+        expect(generator).not_to receive(:generate_or_update_latest_invoice)
+        expect(order).not_to receive(:invoices)
+        expect(renderer).to receive(:render_to_string).with(order)
+        email.deliver_now
+      end
+    end
+
+    context "When invoices feature is enabled" do
+      before do
+        Flipper.enable(:invoices)
+      end
+      it "should call the invoice renderer with invoice's presenter as argument" do
+        expect(generator).to receive(:generate_or_update_latest_invoice)
+        expect(order).to receive(:invoices).and_return([invoice])
+        expect(renderer).to receive(:render_to_string).with(invoice.presenter)
+        email.deliver_now
+      end
+    end
+  end
 end