Add Priority to Promotion Actions

Similar to the scenario of multiple promotions, we need to account for
multiple actions on the same promotion. Also here we need to sort by
explicit priority when allowing cumulative promotions.

core/app/models/spree/promotion_action.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'spree/preferences/persistable'
+require "spree/preferences/persistable"
 
 module Spree
   # Base class for all types of promotion action.
@@ -11,7 +11,9 @@ class PromotionAction < Spree::Base
     include Spree::Preferences::Persistable
     include Spree::SoftDeletable
 
-    belongs_to :promotion, class_name: 'Spree::Promotion', inverse_of: :promotion_actions, optional: true
+    acts_as_list column: :priority, scope: :promotion_id
+
+    belongs_to :promotion, class_name: "Spree::Promotion", inverse_of: :promotion_actions, optional: true
 
     has_many :line_item_discounts, class_name: "Spree::LineItemDiscount", inverse_of: :promotion_action
     has_many :shipment_discounts, class_name: "Spree::ShipmentDiscount", inverse_of: :promotion_action
@@ -26,7 +28,7 @@ class PromotionAction < Spree::Base
     #
     # @note This method should be overriden in subclassses.
     def perform(_options = {})
-      raise 'perform should be implemented in a sub-class of PromotionAction'
+      raise "perform should be implemented in a sub-class of PromotionAction"
     end
 
     # Removes the action from an order
@@ -36,7 +38,7 @@ def perform(_options = {})
     # @param order [Spree::Order] the order to remove the action from
     # @return [void]
     def remove_from(_order)
-      raise 'remove_from should be implemented in a sub-class of PromotionAction'
+      raise "remove_from should be implemented in a sub-class of PromotionAction"
     end
 
     def to_partial_path

core/db/migrate/20220329104242_add_priority_to_spree_promotion_actions.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class AddPriorityToSpreePromotionActions < ActiveRecord::Migration[5.2]
+  def up
+    add_column :spree_promotion_actions, :priority, :integer
+    sql = <<~SQL
+      UPDATE spree_promotion_actions
+      SET priority = id
+    SQL
+    execute(sql)
+    change_column :spree_promotion_actions, :priority, :integer, null: false
+  end
+
+  def down
+    remove_column :spree_promotion_actions, :priority, :integer, null: false
+  end
+end