Add util to get the related objects to delete

oscarapi/serializers/utils.py

@@ -7,9 +7,14 @@
 
 import oscar.models.fields
 
+from oscarapi.utils.loading import get_api_class
 from oscarapi.utils.exists import construct_id_filter
 from .fields import ImageUrlField
 
+get_related_objects_to_delete = get_api_class(
+    "utils.exists", "get_related_objects_to_delete"
+)
+
 logger = logging.getLogger(__name__)
 
 
@@ -57,7 +62,10 @@ def update_relation(self, name, manager, values):
             # the product class changing for example, lots of attributes would become
             # obsolete.
             current_pks = [p.pk for p in updated_values]
-            manager.exclude(pk__in=current_pks).delete()
+            related_objects_to_delete = get_related_objects_to_delete(
+                name, manager, current_pks
+            )
+            related_objects_to_delete.delete()
         else:
             manager.set(updated_values)
 

oscarapi/utils/exists.py

@@ -93,3 +93,8 @@ def bound_unique_together_get_or_create(bound_queryset, datum):
 
 def bound_unique_together_get_or_create_multiple(bound_queryset, data):
     return [bound_unique_together_get_or_create(bound_queryset, date) for date in data]
+
+
+# pylint: disable=W0613
+def get_related_objects_to_delete(name, manager, current_pks):
+    return manager.exclude(pk__in=current_pks)