Add missing association between promo codes and order promotions

This fixes the foreign key constraint error when deleting a promotion
code that has been applied to an order.

This will, of course, change the applicability of the promotion in
question for that particular order. If the order is shipped, no worries
though: The adjustments will stay as our service object will not try to
recalculate. If the order is not shipped, the promotion will stop being
applied. However, if you as a user deleted the promotion or the
promotion code, that's probably what you want.

Fixes #94

friendly_promotions/app/models/solidus_friendly_promotions/promotion_code.rb

@@ -4,6 +4,8 @@ module SolidusFriendlyPromotions
   class PromotionCode < Spree::Base
     belongs_to :promotion, inverse_of: :codes
     belongs_to :promotion_code_batch, inverse_of: :promotion_codes, optional: true
+
+    has_many :order_promotions, class_name: "SolidusFriendlyPromotions::OrderPromotion", dependent: :destroy
     has_many :adjustments, class_name: "Spree::Adjustment"
 
     before_validation :normalize_code

friendly_promotions/spec/models/solidus_friendly_promotions/promotion_code_spec.rb

@@ -3,6 +3,12 @@
 require "spec_helper"
 
 RSpec.describe SolidusFriendlyPromotions::PromotionCode do
+  let(:promotion) { create(:friendly_promotion) }
+  subject { create(:friendly_promotion_code, promotion: promotion) }
+
+  it { is_expected.to belong_to(:promotion) }
+  it { is_expected.to have_many(:order_promotions).class_name("SolidusFriendlyPromotions::OrderPromotion").dependent(:destroy) }
+
   context "callbacks" do
     subject { promotion_code.save }
 
@@ -287,4 +293,19 @@
     }.to raise_error ActiveRecord::RecordInvalid,
       "Validation failed: Could not create promotion code on promotion that apply automatically"
   end
+
+  describe "#destroy" do
+    subject { promotion_code.destroy }
+
+    let(:promotion_code) { create(:friendly_promotion_code) }
+    let(:order) { create(:order_with_line_items) }
+
+    before do
+      order.friendly_order_promotions.create(promotion: promotion_code.promotion, promotion_code: promotion_code)
+    end
+
+    it "destroys the order_promotion" do
+      expect { subject }.to change { SolidusFriendlyPromotions::OrderPromotion.count }.by(-1)
+    end
+  end
 end