Add integration spec

This integration spec comes from the experimental PR
solidusio#4296. That PR will very
probably not be merged, so I'm moving it here as a vantage point.

friendly_promotions/spec/models/promotion/integration_spec.rb

@@ -0,0 +1,85 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe "Promotion System" do
+  context "A promotion that creates line item adjustments" do
+    let(:shirt) { create(:product) }
+    let(:pants) { create(:product) }
+    let(:promotion) { create(:promotion, name: "20% off Shirts", apply_automatically: true) }
+    let(:order) { create(:order) }
+
+    before do
+      promotion.rules << rule
+      promotion.actions << action
+      order.contents.add(shirt.master, 1)
+      order.contents.add(pants.master, 1)
+    end
+
+    context "with an order-level rule" do
+      let(:rule) { Spree::Promotion::Rules::Product.new(products: [shirt]) }
+
+      context "with an order level action" do
+        let(:calculator) { Spree::Calculator::FlatPercentItemTotal.new(preferred_flat_percent: 20) }
+        let(:action) { Spree::Promotion::Actions::CreateAdjustment.new(calculator: calculator) }
+
+        it "creates one order-level adjustment" do
+          expect(order.adjustments.length).to eq(1)
+          expect(order.total).to eq(31.98)
+          expect(order.item_total).to eq(39.98)
+          # This is wrong! But order level adjustments can't work any other way
+          expect(order.item_total_before_tax).to eq(39.98)
+          expect(order.line_items.flat_map(&:adjustments)).to be_empty
+        end
+      end
+
+      context "with an line item level action" do
+        let(:calculator) { Spree::Calculator::PercentOnLineItem.new(preferred_percent: 20) }
+        let(:action) { Spree::Promotion::Actions::CreateItemAdjustments.new(calculator: calculator) }
+
+        it "creates one order-level adjustment" do
+          pending
+          expect(order.adjustments).to be_empty
+          expect(order.total).to eq(31.98)
+          expect(order.item_total).to eq(39.98)
+          expect(order.item_total_before_tax).to eq(31.98)
+          expect(order.line_items.flat_map(&:adjustments).length).to eq(2)
+        end
+      end
+    end
+
+    context "with a line-item level rule" do
+      let(:rule) { Spree::Promotion::Rules::LineItemProduct.new(products: [shirt]) }
+
+      context "with an order level action" do
+        let(:calculator) { Spree::Calculator::FlatPercentItemTotal.new(preferred_flat_percent: 20) }
+        let(:action) { Spree::Promotion::Actions::CreateAdjustment.new(calculator: calculator) }
+
+        it "creates one order-level adjustment" do
+          pending
+          # Whoops - this works because line item level rules don't affect order-level actions :(
+          expect(order.adjustments.length).to eq(1)
+          expect(order.total).to eq(31.98)
+          expect(order.item_total).to eq(39.98)
+          # This is wrong! But order level adjustments can't work any other way
+          expect(order.item_total_before_tax).to eq(39.98)
+          expect(order.line_items.flat_map(&:adjustments)).to be_empty
+        end
+      end
+
+      context "with an line item level action" do
+        let(:calculator) { Spree::Calculator::PercentOnLineItem.new(preferred_percent: 20) }
+        let(:action) { Spree::Promotion::Actions::CreateItemAdjustments.new(calculator: calculator) }
+
+        it "creates one line item level adjustment" do
+          pending
+          expect(order.adjustments).to be_empty
+          expect(order.total).to eq(35.98)
+          expect(order.item_total).to eq(39.98)
+          expect(order.item_total_before_tax).to eq(35.98)
+          expect(order.line_items.flat_map(&:adjustments).length).to eq(1)
+        end
+      end
+    end
+  end
+end