Extract methods related to persisted preferences to module

This removes even more code from Spree::Base, and groups the few lines
of code that are needed for persisted preferences in one module.

core/app/models/spree/base.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'spree/preferences/persistable'
+
 class Spree::Base < ActiveRecord::Base
   include Spree::Preferences::Preferable
   include Spree::Core::Permalinks
@@ -14,26 +16,18 @@ def preferences
         following to lines to your class:
         ```
         class #{self.class.name}
-          serialize :preferences, Hash
-          after_initialize :initialize_preference_defaults
+          include Spree::Preferences::Persistable
           ...
         end
         ```
       WARN
-      self.class.serialize :preferences, Hash
-      self.class.after_initialize :initialize_preference_defaults
+      self.class.include Spree::Preferences::Persistable
       value ? YAML.safe_load(value, [Symbol]) : {}
     else
       value
     end
   end
 
-  def initialize_preference_defaults
-    if has_attribute?(:preferences)
-      self.preferences = default_preferences.merge(preferences)
-    end
-  end
-
   if Kaminari.config.page_method_name != :page
     def self.page(num)
       Spree::Deprecation.warn \

core/app/models/spree/calculator.rb

@@ -1,9 +1,10 @@
 # frozen_string_literal: true
 
+require 'spree/preferences/persistable'
+
 module Spree
   class Calculator < Spree::Base
-    serialize :preferences, Hash
-    after_initialize :initialize_preference_defaults
+    include Spree::Preferences::Persistable
 
     belongs_to :calculable, polymorphic: true, optional: true
 

core/app/models/spree/payment_method.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require 'spree/preferences/persistable'
 require 'spree/preferences/statically_configurable'
 
 module Spree
@@ -11,8 +12,7 @@ module Spree
   # This class is not meant to be instantiated. Please create instances of concrete payment methods.
   #
   class PaymentMethod < Spree::Base
-    serialize :preferences, Hash
-    after_initialize :initialize_preference_defaults
+    include Spree::Preferences::Persistable
 
     preference :server, :string, default: 'test'
     preference :test_mode, :boolean, default: true

core/app/models/spree/promotion_action.rb

@@ -1,14 +1,14 @@
 # frozen_string_literal: true
 
+require 'spree/preferences/persistable'
+
 module Spree
   # Base class for all types of promotion action.
   #
   # PromotionActions perform the necessary tasks when a promotion is activated
   # by an event and determined to be eligible.
   class PromotionAction < Spree::Base
-    serialize :preferences, Hash
-    after_initialize :initialize_preference_defaults
-
+    include Spree::Preferences::Persistable
     include Spree::SoftDeletable
 
     belongs_to :promotion, class_name: 'Spree::Promotion', inverse_of: :promotion_actions, optional: true

core/app/models/spree/promotion_rule.rb

@@ -1,10 +1,11 @@
 # frozen_string_literal: true
 
+require 'spree/preferences/persistable'
+
 module Spree
   # Base class for all promotion rules
   class PromotionRule < Spree::Base
-    serialize :preferences, Hash
-    after_initialize :initialize_preference_defaults
+    include Spree::Preferences::Persistable
 
     belongs_to :promotion, class_name: 'Spree::Promotion', inverse_of: :promotion_rules, optional: true
 

core/lib/spree/preferences/persistable.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Spree
+  module Preferences
+    module Persistable
+      def self.included(klass)
+        klass.serialize :preferences, Hash
+        klass.after_initialize :initialize_preference_defaults
+      end
+
+      private
+
+      def initialize_preference_defaults
+        if has_attribute?(:preferences)
+          self.preferences = default_preferences.merge(preferences)
+        end
+      end
+    end
+  end
+end