Make this test postgres-compatible

We don't run the testsuite on postgres yet, but we probably should.

example/tests/test_fields.py

@@ -1,34 +1,52 @@
 # -*- coding: utf-8
 from __future__ import unicode_literals
 
+import decimal
+
+from django.contrib.auth import get_user_model
 from django.db import models
 from django.test import TestCase
 
-from shop.models.defaults.cart_item import CartItem
 from shop.models.fields import JSONField
 
+from myshop.models import Cart, CartItem, Customer
+from myshop.models.manufacturer import Manufacturer
+from myshop.models.polymorphic.commodity import Commodity
+
 
 class JsonModel(models.Model):
 
     class Meta:
         app_label = 'JsonModel'
 
     json = JSONField()
-    default_json = JSONField(default={"check": 12})
+    default_json = JSONField(default=lambda: {"check": 12})
 
 
 class JSONFieldTest(TestCase):
     """JSONField Tests"""
 
-    fixtures = ['myshop-polymorphic.json']
-
     def setUp(self):
         super(JSONFieldTest, self).setUp()
-        self.sample = CartItem.objects.get(id=8)
+        user = get_user_model().objects.create()
+        customer = Customer.objects.create(number=1, user=user)
+        manufacturer = Manufacturer.objects.create()
+        product = Commodity.objects.create(
+            product_code="testproduct",
+            unit_price=decimal.Decimal("0"),
+            order=1,
+            manufacturer=manufacturer
+        )
+        cart = Cart.objects.create(customer=customer)
+        self.sample = CartItem.objects.create(
+            quantity=1,
+            cart=cart,
+            product=product,
+            extra={'product_code': 'foo'},
+        )
         self.assertIsNotNone(self.sample)
 
     def test_json_field_create(self):
         """Test saving a JSON object in our JSONField"""
-        extra = {"product_code":"1121"}
-
+        extra = {'product_code': 'foo'}
         self.assertEqual(self.sample.extra, extra)