Polymorphically associate SemanticLinks to variant

app/models/semantic_link.rb

@@ -2,6 +2,7 @@
 
 # Link a Spree::Variant to an external DFC SuppliedProduct.
 class SemanticLink < ApplicationRecord
+  belongs_to :subject, polymorphic: true
   belongs_to :variant, class_name: "Spree::Variant"
 
   validates :semantic_id, presence: true

db/migrate/20241030025540_copy_subject_on_semantic_links.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class CopySubjectOnSemanticLinks < ActiveRecord::Migration[7.0]
+  def up
+    execute <<~SQL.squish
+      UPDATE semantic_links SET
+        subject_id   = variant_id,
+        subject_type = 'Spree::Variant'
+    SQL
+  end
+end

spec/migrations/20241030025540_copy_subject_on_semantic_links_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require_relative '../../db/migrate/20241030025540_copy_subject_on_semantic_links'
+
+RSpec.describe CopySubjectOnSemanticLinks do
+  describe "#up" do
+    let(:original_variant) { create(:variant) }
+    let(:dummy_variant) { create(:variant) }
+
+    it "copies the original data" do
+      link = SemanticLink.create!(
+        variant: original_variant,
+        subject: dummy_variant, # This would be NULL when migration runs.
+        semantic_id: "some-url",
+      )
+
+      expect { subject.up }.to change {
+        link.reload.subject
+      }.from(dummy_variant).to(original_variant)
+    end
+  end
+end

spec/models/semantic_link_spec.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe SemanticLink, type: :model do
+  it { is_expected.to belong_to :subject }
   it { is_expected.to belong_to :variant }
   it { is_expected.to validate_presence_of(:semantic_id) }
 end