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 +1,32 @@
+# frozen_string_literal: true
+
+class AddFkToClassifications < ActiveRecord::Migration[7.0]
+  def up
+    # Uncomment the following code to remove orphaned records if this migration fails
+    #
+    # say_with_time "Removing orphaned orders (no corresponding user)" do
+    #   Spree::Classification.left_joins(:product).where(spree_products: { id: nil }).delete_all
+    #   Spree::Classification.left_joins(:taxon).where(spree_taxons: { id: nil }).delete_all
+    # end
+
+    add_foreign_key :spree_products_taxons, :spree_products, column: :product_id
+    add_foreign_key :spree_products_taxons, :spree_taxons, column: :taxon_id
+  rescue ActiveRecord::StatementInvalid => e
+    if e.cause.is_a?(PG::ForeignKeyViolation) || e.cause.is_a?(Mysql2::Error) || e.cause.is_a?(SQLite3::ConstraintException)
+      Rails.logger.warn <<~MSG
+        ⚠️ Foreign key constraint failed when adding :spree_products_taxons => :spree_products.
+        ⚠️ Foreign key constraint failed when adding :spree_products_taxons => :spree_taxons.
+        To fix this:
+          1. Uncomment the code that removes orphaned records.
+          2. Rerun the migration.
+        Offending error: #{e.cause.class} - #{e.cause.message}
+      MSG
+    end
+    raise
+  end
+
+  def down
+    remove_foreign_key :spree_products_taxons, :spree_products, column: :product_id
+    remove_foreign_key :spree_products_taxons, :spree_taxons, column: :taxon_id
+  end
+end