[Fix] Resolve product class updating to None in Django 3.2 (#12)

* fix 🐛 resolve product class updating to None in django 3.2

* refactor 📦 use existing method from base.Model

oscar_odin/mappings/context.py

@@ -182,6 +182,9 @@ def bulk_update_or_create_instances(self, instances):
 
         fields = self.get_fields_to_update(self.Model)
         if fields is not None:
+            for instance in instances_to_update:
+                # This should be removed once support for django 3.2 is dropped
+                instance._prepare_related_fields_for_save("bulk_update")
             self.Model.objects.bulk_update(instances_to_update, fields=fields)
 
     def bulk_update_or_create_one_to_many(self):

oscar_odin/utils.py

@@ -1,13 +1,10 @@
+from collections import defaultdict
 import contextlib
 import time
 import math
 
-from django.db import connection, reset_queries
-
-from collections import defaultdict
-
-from django.db.models import Model, ManyToManyField, ForeignKey, Q
-from django.db import connections
+from django.db import connection, connections, reset_queries
+from django.db.models import Q
 
 
 def get_filters(instances, field_names):