Classifications: Add foreign keys

This join table needs both references to be pointing to valid records.

A uniqueness index for the taxon id in this table is already present.

Author: mamhoff

core/app/models/spree/classification.rb

@@ -4,8 +4,8 @@ module Spree
   class Classification < Spree::Base
     self.table_name = 'spree_products_taxons'
     acts_as_list scope: :taxon
-    belongs_to :product, class_name: "Spree::Product", inverse_of: :classifications, touch: true, optional: true
-    belongs_to :taxon, class_name: "Spree::Taxon", inverse_of: :classifications, touch: true, optional: true
+    belongs_to :product, class_name: "Spree::Product", inverse_of: :classifications, touch: true
+    belongs_to :taxon, class_name: "Spree::Taxon", inverse_of: :classifications, touch: true
 
     # For https://github.com/spree/spree/issues/3494
     validates :taxon_id, uniqueness: { scope: :product_id, message: :already_linked }

core/db/migrate/20250604073219_add_fk_to_classifications.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class AddFkToClassifications < ActiveRecord::Migration[7.0]
+  def change
+    add_foreign_key :spree_products_taxons, :spree_products, column: :product_id
+    add_foreign_key :spree_products_taxons, :spree_taxons, column: :taxon_id
+  end
+end