Extend PaymentMethod with calculated_adjustments

app/models/spree/payment_method_decorator.rb

@@ -4,6 +4,10 @@ Spree::PaymentMethod.class_eval do
 
   attr_accessible :distributor_ids
 
+  calculated_adjustments
+
+  after_initialize :init
+
   validates :distributors, presence: { message: "^At least one hub must be selected" }
 
   # -- Scopes
@@ -31,6 +35,11 @@ Spree::PaymentMethod.class_eval do
     where('spree_payment_methods.environment=? OR spree_payment_methods.environment=? OR spree_payment_methods.environment IS NULL', Rails.env, '')
   }
 
+  def init
+    self.class.calculated_adjustments unless reflections.keys.include? :calculator
+    self.calculator ||= Spree::Calculator::FlatRate.new(preferred_amount: 0)
+  end
+
   def has_distributor?(distributor)
     self.distributors.include?(distributor)
   end

spec/models/spree/payment_method_spec.rb

@@ -25,5 +25,13 @@ module Spree
       # Testing else condition
       Spree::Gateway::BogusSimple.clean_name.should == "BogusSimple"
     end
+
+    it "computes the amount of fees" do
+      pickup = create(:payment_method, name: 'pickup')
+      order = create(:order)
+      expect(pickup.compute_amount(order)).to eq 0
+      delivery = create(:payment_method, name: 'delivery', calculator: Calculator::FlatRate.new(preferred_amount: 10))
+      expect(delivery.compute_amount(order)).to eq 10
+    end
   end
 end