In current_order_serializer, serialize shipping_method_id from order.shipping_method.id. Adapted respective spec.

app/serializers/api/current_order_serializer.rb

@@ -12,6 +12,10 @@ class Api::CurrentOrderSerializer < ActiveModel::Serializer
     object.payments.first.andand.payment_method_id
   end
 
+  def shipping_method_id
+    object.shipping_method.andand.id
+  end
+
   def display_total
     object.display_total.money.to_f
   end

spec/serializers/current_order_serializer_spec.rb

@@ -1,19 +1,35 @@
+require 'spec_helper'
+
 describe Api::CurrentOrderSerializer do
   let(:distributor) { create(:distributor_enterprise) }
-  let(:oc) { create(:simple_order_cycle) }
-  let(:li) { create(:line_item, variant: create(:variant)) }
-  let(:order) { create(:order, line_items: [li]) }
-  let(:serializer) { Api::CurrentOrderSerializer.new(order, current_distributor: distributor, current_order_cycle: oc ).to_json }
+  let(:order_cycle) { create(:simple_order_cycle) }
+  let(:line_item) { create(:line_item, variant: create(:variant)) }
+  let(:order) { create(:order, line_items: [line_item]) }
+  let(:serializer) { Api::CurrentOrderSerializer.new(order, current_distributor: distributor, current_order_cycle: order_cycle).to_json }
 
   it "serializers the current order" do
-    serializer.should match order.id.to_s
+    expect(serializer).to match order.id.to_s
   end
 
   it "includes line items" do
-    serializer.should match li.id.to_s
+    expect(serializer).to match line_item.id.to_s
   end
 
   it "includes variants of line items" do
-    serializer.should match li.variant.name
+    expect(serializer).to match line_item.variant.name
+  end
+
+  context 'when there is no shipment' do
+    it 'includes the shipping method of the order' do
+      expect(serializer).to match('\"shipping_method_id\":null')
+    end
+  end
+
+  context 'when there is a shipment' do
+    before { create(:shipment, order: order) }
+
+    it 'includes the shipping method of the order' do
+      expect(serializer).to match("\"shipping_method_id\":#{order.shipping_method.id}")
+    end
   end
 end