Don't persist models to test serializer

Serializers don't care whether or not the objects they go through are
persisted in DB. They just convert them.

spec/serializers/api/current_order_serializer_spec.rb

@@ -3,11 +3,17 @@
 require 'spec_helper'
 
 describe Api::CurrentOrderSerializer do
-  let(:distributor) { create(:distributor_enterprise) }
-  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 }
+  let(:distributor) { build(:distributor_enterprise) }
+  let(:order_cycle) { build(:simple_order_cycle) }
+  let(:line_item) { build(:line_item, variant: create(:variant)) }
+  let(:order) { build(:order, line_items: [line_item]) }
+  let(:serializer) do
+    Api::CurrentOrderSerializer.new(
+      order,
+      current_distributor: distributor,
+      current_order_cycle: order_cycle
+    ).to_json
+  end
 
   it "serializers the current order" do
     expect(serializer).to match order.id.to_s
@@ -28,7 +34,11 @@ describe Api::CurrentOrderSerializer do
   end
 
   context 'when there is a shipment' do
-    before { create(:shipment, order: order) }
+    let(:shipping_method) { build(:shipping_method) }
+
+    before do
+      allow(order).to receive(:shipping_method).and_return(shipping_method)
+    end
 
     it 'includes the shipping method of the order' do
       expect(serializer).to match("\"shipping_method_id\":#{order.shipping_method.id}")