diff --git a/app/models/concerns/order_shipping_method.rb b/app/models/concerns/order_shipping_method.rb
new file mode 100644
index 00000000000..75e729f24e9
--- /dev/null
+++ b/app/models/concerns/order_shipping_method.rb
@@ -0,0 +1,21 @@
+require 'active_support/concern'
+
+# This module is an adapter for OFN to work with Spree 2 code.
+#
+# Although Spree 2 supports multiple shipments per order, in OFN we have only one shipment per order.
+# A shipment is associated to a shipping_method through a selected shipping_rate.
+# See https://github.com/openfoodfoundation/openfoodnetwork/wiki/Spree-Upgrade:-Migration-to-multiple-shipments
+# for details.
+#
+# Methods in this module may become deprecated.
+module OrderShippingMethod
+  extend ActiveSupport::Concern
+
+  # Returns the shipping method of the first and only shipment in the order
+  #
+  # @return [ShippingMethod]
+  def shipping_method
+    return if shipments.empty?
+    shipments.first.shipping_method
+  end
+end
diff --git a/app/models/spree/order_decorator.rb b/app/models/spree/order_decorator.rb
index 458742ebf4a..8a88078b05d 100644
--- a/app/models/spree/order_decorator.rb
+++ b/app/models/spree/order_decorator.rb
@@ -2,12 +2,15 @@
 require 'open_food_network/distribution_change_validator'
 require 'open_food_network/feature_toggle'
 require 'open_food_network/tag_rule_applicator'
+require 'concerns/order_shipping_method'
 
 ActiveSupport::Notifications.subscribe('spree.order.contents_changed') do |name, start, finish, id, payload|
   payload[:order].reload.update_distribution_charge!
 end
 
 Spree::Order.class_eval do
+  include OrderShippingMethod
+
   belongs_to :order_cycle
   belongs_to :distributor, class_name: 'Enterprise'
   belongs_to :customer
diff --git a/db/migrate/20180910155506_add_uniqueness_of_order_id_to_spree_shipments.rb b/db/migrate/20180910155506_add_uniqueness_of_order_id_to_spree_shipments.rb
new file mode 100644
index 00000000000..21ad966ad49
--- /dev/null
+++ b/db/migrate/20180910155506_add_uniqueness_of_order_id_to_spree_shipments.rb
@@ -0,0 +1,6 @@
+class AddUniquenessOfOrderIdToSpreeShipments < ActiveRecord::Migration
+  def change
+    remove_index :spree_shipments, :order_id
+    add_index :spree_shipments, :order_id, unique: true
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 1d7f1677246..007691a87ca 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20180812214434) do
+ActiveRecord::Schema.define(:version => 20180910155506) do
 
   create_table "account_invoices", :force => true do |t|
     t.integer  "user_id",    :null => false
@@ -876,7 +876,7 @@
   end
 
   add_index "spree_shipments", ["number"], :name => "index_shipments_on_number"
-  add_index "spree_shipments", ["order_id"], :name => "index_spree_shipments_on_order_id"
+  add_index "spree_shipments", ["order_id"], :name => "index_spree_shipments_on_order_id", :unique => true
 
   create_table "spree_shipping_categories", :force => true do |t|
     t.string   "name"
diff --git a/spec/factories.rb b/spec/factories.rb
index 4824f5be6b4..9c0a88e3181 100644
--- a/spec/factories.rb
+++ b/spec/factories.rb
@@ -280,16 +280,6 @@
     after(:create) { |c| c.set_preference(:per_kg, 0.5); c.save! }
   end
 
-  factory :shipping_method_with_flat_rate, parent: :shipping_method do
-    calculator { Spree::Calculator::FlatRate.new(preferred_amount: 50.0) }
-  end
-
-  factory :shipment_with_flat_rate, parent: :shipment do
-    after(:create) do |shipment|
-      shipment.add_shipping_method(create(:shipping_method_with_flat_rate), true)
-    end
-  end
-
   factory :order_with_totals_and_distribution, :parent => :order do #possibly called :order_with_line_items in newer Spree
     distributor { create(:distributor_enterprise) }
     order_cycle { create(:simple_order_cycle) }
diff --git a/spec/models/concerns/order_shipping_method_spec.rb b/spec/models/concerns/order_shipping_method_spec.rb
new file mode 100644
index 00000000000..86c178e1158
--- /dev/null
+++ b/spec/models/concerns/order_shipping_method_spec.rb
@@ -0,0 +1,22 @@
+require 'spec_helper'
+
+describe OrderShippingMethod do
+  let(:order) { create(:order) }
+
+  describe '#shipping_method' do
+    context 'when order has no shipments' do
+      it 'returns nil' do
+        expect(order.shipping_method).to be_nil
+      end
+    end
+
+    context 'when order has single shipment' do
+      it 'returns the shipments shipping_method' do
+        shipment = create(:shipment_with_flat_rate)
+        order.shipments = [shipment]
+
+        expect(order.shipping_method).to eq shipment.shipping_method
+      end
+    end
+  end
+end