[Spree upgrade] Workaround Rails inheritance bug in Spree::Gateway

config/initializers/spree.rb

@@ -8,6 +8,16 @@
 
 require 'spree/product_filters'
 
+# Due to a bug in ActiveRecord we need to load the tagging code in Gateway which
+# should have inherited it from its parent PaymentMethod.
+# We have to call it before loading the PaymentMethod decorator because the
+# tagging code won't load twice within the inheritance chain.
+# https://github.com/openfoodfoundation/openfoodnetwork/issues/3121
+Spree::Gateway.class_eval do
+  acts_as_taggable
+  attr_accessible :tag_list
+end
+
 require "#{Rails.root}/app/models/spree/payment_method_decorator"
 require "#{Rails.root}/app/models/spree/gateway_decorator"
 

spec/models/spree/gateway_tagging_spec.rb

@@ -0,0 +1,56 @@
+require "spec_helper"
+
+# We extended Spree::PaymentMethod to be taggable. Unfortunately, an inheritance
+# bug prevented the taggable code to be passed on to the descendants of
+# PaymentMethod. We fixed that in config/initializers/spree.rb.
+#
+# This spec tests several descendants for their taggability. The tests are in
+# a separate file, because they cover one aspect of several classes.
+shared_examples "taggable" do |expected_taggable_type|
+  it "provides a tag list" do
+    expect(subject.tag_list).to eq []
+  end
+
+  it "stores tags for the root taggable type" do
+    subject.tag_list.add("one")
+    subject.save!
+
+    expect(subject.taggings.last.taggable_type).to eq expected_taggable_type
+  end
+end
+
+module Spree
+  describe "PaymentMethod and descendants" do
+
+    let(:shop) { create(:enterprise) }
+    let(:valid_subject) do
+      # Supply required parameters so that it can be saved to attach taggings.
+      described_class.new(
+        name: "Some payment method",
+        distributor_ids: [shop.id]
+      )
+    end
+    subject { valid_subject }
+
+    describe PaymentMethod do
+      it_behaves_like "taggable", "Spree::PaymentMethod"
+    end
+
+    describe Gateway do
+      it_behaves_like "taggable", "Spree::PaymentMethod"
+    end
+
+    describe Gateway::PayPalExpress do
+      it_behaves_like "taggable", "Spree::PaymentMethod"
+    end
+
+    describe Gateway::StripeConnect do
+      subject do
+        # StripeConnect needs an owner to be valid.
+        valid_subject.tap { |m| m.preferred_enterprise_id = shop.id }
+      end
+
+      it_behaves_like "taggable", "Spree::PaymentMethod"
+    end
+  end
+end