CreateVariantLinks [migration]

Tried using the rails generator, but as usual it was a waste of time becuase it doesn't handle unusual cases.

I found more good guidance from that stackoverflow post:
> why are you worrying about your indexes? Build your app!

Something's not right in the model, see next commit.

app/models/spree/variant.rb

@@ -72,6 +72,10 @@ module Spree
     has_many :semantic_links, as: :subject, dependent: :delete_all
     has_many :supplier_properties, through: :supplier, source: :properties
 
+    has_many :variant_links, dependent: :delete_all
+    has_many :source_variants, through: :variant_links
+    has_many :target_variants, through: :variant_links
+
     localize_number :price, :weight
 
     validates_lengths_from_database

app/models/variant_link.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class VariantLink < ApplicationRecord
+  belongs_to :source_variant, class_name: 'Spree::Variant', touch: true
+  belongs_to :target_variant, class_name: 'Spree::Variant', touch: true
+end

db/migrate/20260211055758_create_variant_links.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class CreateVariantLinks < ActiveRecord::Migration[7.1]
+  def change
+    # Create a join table to join two variants. One is the source of the other.
+    # Primary key index ensures uniqueness and assists querying. target_variant_id is the most
+    # likely subject and so is first in the index.
+    # An additional index for source_variant is also included because it may be helpful
+    # (https://stackoverflow.com/questions/10790518/best-sql-indexes-for-join-table).
+    create_table :variant_links, primary_key: [:target_variant_id, :source_variant_id] do |t|
+      t.integer :source_variant_id, null: false, index: true
+      t.integer :target_variant_id, null: false
+
+      t.timestamps
+    end
+    add_foreign_key :variant_links, :spree_variants, column: :source_variant_id
+    add_foreign_key :variant_links, :spree_variants, column: :target_variant_id
+  end
+end

db/schema.rb

@@ -258,8 +258,8 @@ ActiveRecord::Schema[7.1].define(version: 2026_03_06_015040) do
     t.text "white_label_logo_link"
     t.boolean "hide_groups_tab", default: false
     t.string "external_billing_id", limit: 128
-    t.boolean "enable_producers_to_edit_orders", default: false, null: false
     t.boolean "show_customer_contacts_to_suppliers", default: false, null: false
+    t.boolean "enable_producers_to_edit_orders", default: false, null: false
     t.index ["address_id"], name: "index_enterprises_on_address_id"
     t.index ["is_primary_producer", "sells"], name: "index_enterprises_on_is_primary_producer_and_sells"
     t.index ["name"], name: "index_enterprises_on_name", unique: true
@@ -1113,6 +1113,14 @@ ActiveRecord::Schema[7.1].define(version: 2026_03_06_015040) do
     t.datetime "updated_at", precision: nil, null: false
   end
 
+  create_table "variant_links", primary_key: ["target_variant_id", "source_variant_id"], force: :cascade do |t|
+    t.integer "source_variant_id", null: false
+    t.integer "target_variant_id", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["source_variant_id"], name: "index_variant_links_on_source_variant_id"
+  end
+
   create_table "variant_overrides", id: :serial, force: :cascade do |t|
     t.integer "variant_id", null: false
     t.integer "hub_id", null: false
@@ -1278,6 +1286,8 @@ ActiveRecord::Schema[7.1].define(version: 2026_03_06_015040) do
   add_foreign_key "subscriptions", "spree_payment_methods", column: "payment_method_id", name: "subscriptions_payment_method_id_fk"
   add_foreign_key "subscriptions", "spree_shipping_methods", column: "shipping_method_id", name: "subscriptions_shipping_method_id_fk"
   add_foreign_key "tag_rules", "enterprises"
+  add_foreign_key "variant_links", "spree_variants", column: "source_variant_id"
+  add_foreign_key "variant_links", "spree_variants", column: "target_variant_id"
   add_foreign_key "variant_overrides", "enterprises", column: "hub_id", name: "variant_overrides_hub_id_fk"
   add_foreign_key "variant_overrides", "spree_variants", column: "variant_id", name: "variant_overrides_variant_id_fk"
   add_foreign_key "vouchers", "enterprises"