Change type and name of basket field on AbstractOrder

It used to be a PositiveIntegerField containing the primary key of the
basket. This was a minor design deficiency; a nullable ForeignKey field
seems like the correct way to model that relation.
It's also been renamed from basket_id to basket to reflect the change.

Migrations unfortunately are non-trivial:
* 0019 adds the new ForeignKey field, but called basket_alt
* 0020 migrates data from the old field to the new field
* 0021 drops the now unnecessary field basket_id
* 0022 renames the basket_alt field to basket

The additional step with naming it basket_alt is necessary because of a
column name conflict. Django will pick a column name of 'basket_id' for
a ForeignKey field named 'basket', which is the same column name the
old PositiveIntegerField had.

docs/source/releases/v0.7.rst

@@ -24,6 +24,10 @@ Minor changes
 Backwards incompatible changes in 0.7
 =====================================
 
+* Order.basket_id was a PositiveIntegerField containing the primary key of the
+  associated basket. It's been refactored to be a nullable ForeignKeyField and
+  is now called basket.
+
 Removal of features deprecated in 0.6
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

oscar/apps/order/abstract_models.py

@@ -21,8 +21,8 @@ class AbstractOrder(models.Model):
 
     # We track the site that each order is placed within
     site = models.ForeignKey('sites.Site', verbose_name=_("Site"))
-    basket_id = models.PositiveIntegerField(
-        _("Basket ID"), null=True, blank=True)
+    basket = models.ForeignKey('basket.Basket', verbose_name=_("Basket"),
+                               null=True, blank=True, on_delete=models.SET_NULL)
 
     # Orders can be anonymous so we don't always have a customer ID
     user = models.ForeignKey(

oscar/apps/order/utils.py

@@ -102,7 +102,7 @@ def create_order_model(self, user, basket, shipping_address,
         """
         Creates an order model.
         """
-        order_data = {'basket_id': basket.id,
+        order_data = {'basket': basket,
                       'number': order_number,
                       'site': Site._default_manager.get_current(),
                       'currency': total.currency,