Gateway providers inherit from decorated Gateway and PaymentMethod classes in production

Achieved by requiring payment method and gateway decorators in Spree initializer

app/models/spree/gateway/pay_pal_express_decorator.rb

@@ -1,9 +0,0 @@
-require_relative '../gateway_decorator'
-
-module Spree
-  class Gateway::PayPalExpress < Gateway
-    # Something odd is happening with class inheritance here, this class (defined in spree_paypal_express gem)
-    # doesn't seem to pick up attr_accessible from the Gateway class, so we redefine the attrs we need here
-    attr_accessible :tag_list
-  end
-end

app/models/spree/gateway_decorator.rb

@@ -1,18 +1,5 @@
 Spree::Gateway.class_eval do
-  acts_as_taggable
-
-  # Due to class load order, when config.cache_classes is enabled (ie. staging and production
-  # environments), this association isn't inherited from PaymentMethod. As a result, creating
-  # payment methods using payment gateways results in:
-  # undefined method `association_class' for nil:NilClass
-  # To avoid that, we redefine this association here.
-
-  has_and_belongs_to_many :distributors, join_table: 'distributors_payment_methods', :class_name => 'Enterprise', foreign_key: 'payment_method_id', association_foreign_key: 'distributor_id'
-
-
   # Default to live
   preference :server, :string, :default => 'live'
   preference :test_mode, :boolean, :default => false
-
-  attr_accessible :tag_list
 end

app/models/spree/payment_method_decorator.rb

@@ -1,7 +1,6 @@
 Spree::PaymentMethod.class_eval do
   acts_as_taggable
 
-  # See gateway_decorator.rb when modifying this association
   has_and_belongs_to_many :distributors, join_table: 'distributors_payment_methods', :class_name => 'Enterprise', association_foreign_key: 'distributor_id'
 
   attr_accessible :distributor_ids, :tag_list
@@ -45,26 +44,20 @@ Spree::PaymentMethod.class_eval do
   def has_distributor?(distributor)
     self.distributors.include?(distributor)
   end
-end
 
-# Ensure that all derived classes also allow distributor_ids
-Spree::Gateway.providers.each do |p|
-  p.attr_accessible :distributor_ids
-  p.instance_eval do
-    def clean_name
-      case name
-      when "Spree::PaymentMethod::Check"
-        "Cash/EFT/etc. (payments for which automatic validation is not required)"
-      when "Spree::Gateway::Migs"
-        "MasterCard Internet Gateway Service (MIGS)"
-      when "Spree::Gateway::Pin"
-        "Pin Payments"
-      when "Spree::Gateway::PayPalExpress"
-        "PayPal Express"
-      else
-        i = name.rindex('::') + 2
-        name[i..-1]
-      end
+  def self.clean_name
+    case name
+    when "Spree::PaymentMethod::Check"
+      "Cash/EFT/etc. (payments for which automatic validation is not required)"
+    when "Spree::Gateway::Migs"
+      "MasterCard Internet Gateway Service (MIGS)"
+    when "Spree::Gateway::Pin"
+      "Pin Payments"
+    when "Spree::Gateway::PayPalExpress"
+      "PayPal Express"
+    else
+      i = name.rindex('::') + 2
+      name[i..-1]
     end
   end
 end

config/initializers/spree.rb

@@ -9,6 +9,9 @@
 
 require 'spree/product_filters'
 
+require "#{Rails.root}/app/models/spree/payment_method_decorator"
+require "#{Rails.root}/app/models/spree/gateway_decorator"
+
 Spree.config do |config|
   config.shipping_instructions = true
   config.address_requires_state = true