Added support for tracking FK ID updates.

src/auditlog/diff.py

@@ -2,7 +2,7 @@
 
 from django.conf import settings
 from django.core.exceptions import ObjectDoesNotExist
-from django.db.models import Model, NOT_PROVIDED, DateTimeField
+from django.db.models import Model, NOT_PROVIDED, DateTimeField, ForeignKey
 from django.utils import timezone
 from django.utils.encoding import smart_text
 
@@ -134,6 +134,14 @@ def model_instance_diff(old, new):
         old_value = get_field_value(old, field)
         new_value = get_field_value(new, field)
 
+        # Handle cases where a models ForeignKey field is saved using the
+        # '*_id' field and not the ForeignKey field itself. In this situation,
+        # the fields will appear to be identical when comparing the objects,
+        # but different when comparing the '*_id' fields.
+        if isinstance(field, ForeignKey) and old_value == new_value:
+            old_value = smart_text(getattr(old, field.attname, None))
+            new_value = smart_text(getattr(new, field.attname, None))
+
         if old_value != new_value:
             diff[field.name] = (smart_text(old_value), smart_text(new_value))
 

src/auditlog_tests/models.py

@@ -66,7 +66,9 @@ class RelatedModel(models.Model):
     A model with a foreign key.
     """
 
-    related = models.ForeignKey(to='self', on_delete=models.CASCADE)
+    related = models.ForeignKey(
+        to='self', on_delete=models.CASCADE,
+        null=True, blank=True)
 
     history = AuditlogHistoryField()
 

src/auditlog_tests/tests.py

@@ -102,6 +102,55 @@ def setUp(self):
         self.obj = ProxyModel.objects.create(text='I am not what you think.')
 
 
+class RelatedModelTest(TestCase):
+    """
+    Test the behaviour of relationships.
+    """
+    def setUp(self):
+        self.obj = RelatedModel.objects.create()
+        self.rel_obj = RelatedModel.objects.create(related=self.obj)
+
+    def test_related(self):
+        # Creation of 'obj' should create single history record.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj).count(), 1)
+        # There is no 'related' object (None) so no history on it.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj.related).count(), 0)
+
+        # Creation of 'rel_obj' should create single history record.
+        self.assertEqual(LogEntry.objects.get_for_object(self.rel_obj).count(), 1)
+        # The 'related' object is 'obj' and should have a single record as well.
+        self.assertEqual(LogEntry.objects.get_for_object(self.rel_obj.related).count(), 1)
+
+        # Set a 'related' object.
+        self.obj.related = self.rel_obj
+        self.obj.save()
+
+        # The 'obj' model now has two history records.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj).count(), 2)
+        # The 'related' object is now 'rel_obj' and has a single history record.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj.related).count(), 1)
+
+        # Wipe out the relationship by changing the '*_id' field.
+        self.obj.related_id = None
+        self.obj.save()
+        self.obj.refresh_from_db()
+
+        # The 'obj' model now has three history records.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj).count(), 3)
+        # The 'related' object is now 'None' so no history records.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj.related).count(), 0)
+
+        # Set the relationship back using the '*_id' field.
+        self.obj.related_id = self.rel_obj.id
+        self.obj.save()
+        self.obj.refresh_from_db()
+
+        # The 'obj' model now has four history records.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj).count(), 4)
+        # The 'related' object is now 'rel_obj' and has a single history record.
+        self.assertEqual(LogEntry.objects.get_for_object(self.obj.related).count(), 1)
+
+
 class ManyRelatedModelTest(TestCase):
     """
     Test the behaviour of a many-to-many relationship.