(feature) Allow AuditlogHistoryField to block cascading deletes (#172)

Fixes #108

Thanks to @andrewwatts for suggesting and researching the approach.

src/auditlog/models.py

@@ -7,7 +7,7 @@
 from django.contrib.contenttypes.fields import GenericRelation
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import FieldDoesNotExist
-from django.db import models
+from django.db import models, DEFAULT_DB_ALIAS
 from django.db.models import QuerySet, Q
 from django.utils import formats, timezone
 from django.utils.encoding import python_2_unicode_compatible, smart_text
@@ -321,9 +321,13 @@ class AuditlogHistoryField(GenericRelation):
 
     :param pk_indexable: Whether the primary key for this model is not an :py:class:`int` or :py:class:`long`.
     :type pk_indexable: bool
+    :param delete_related: By default, including a generic relation into a model will cause all related objects to be
+        cascade-deleted when the parent object is deleted. Passing False to this overrides this behavior, retaining
+        the full auditlog history for the object. Defaults to True, because that's Django's default behavior.
+    :type delete_related: bool
     """
 
-    def __init__(self, pk_indexable=True, **kwargs):
+    def __init__(self, pk_indexable=True, delete_related=True, **kwargs):
         kwargs['to'] = LogEntry
 
         if pk_indexable:
@@ -332,8 +336,22 @@ def __init__(self, pk_indexable=True, **kwargs):
             kwargs['object_id_field'] = 'object_pk'
 
         kwargs['content_type_field'] = 'content_type'
+        self.delete_related = delete_related
         super(AuditlogHistoryField, self).__init__(**kwargs)
 
+    def bulk_related_objects(self, objs, using=DEFAULT_DB_ALIAS):
+        """
+        Return all objects related to ``objs`` via this ``GenericRelation``.
+        """
+        if self.delete_related:
+            return super(AuditlogHistoryField, self).bulk_related_objects(objs, using)
+
+        # When deleting, Collector.collect() finds related objects using this
+        # method.  However, because we don't want to delete these related
+        # objects, we simply return an empty list.
+        return []
+
+
 # South compatibility for AuditlogHistoryField
 try:
     from south.modelsinspector import add_introspection_rules

src/auditlog_tests/models.py

@@ -209,6 +209,12 @@ class PostgresArrayFieldModel(models.Model):
     history = AuditlogHistoryField()
 
 
+class NoDeleteHistoryModel(models.Model):
+    integer = models.IntegerField(blank=True, null=True)
+
+    history = AuditlogHistoryField(delete_related=False)
+
+
 auditlog.register(AltPrimaryKeyModel)
 auditlog.register(UUIDPrimaryKeyModel)
 auditlog.register(ProxyModel)
@@ -222,3 +228,4 @@ class PostgresArrayFieldModel(models.Model):
 auditlog.register(ChoicesFieldModel)
 auditlog.register(CharfieldTextfieldModel)
 auditlog.register(PostgresArrayFieldModel)
+auditlog.register(NoDeleteHistoryModel)

src/auditlog_tests/tests.py

@@ -16,7 +16,7 @@
 from auditlog_tests.models import SimpleModel, AltPrimaryKeyModel, UUIDPrimaryKeyModel, \
     ProxyModel, SimpleIncludeModel, SimpleExcludeModel, SimpleMappingModel, RelatedModel, \
     ManyRelatedModel, AdditionalDataIncludedModel, DateTimeFieldModel, ChoicesFieldModel, \
-    CharfieldTextfieldModel, PostgresArrayFieldModel
+    CharfieldTextfieldModel, PostgresArrayFieldModel, NoDeleteHistoryModel
 from auditlog import compat
 
 
@@ -663,3 +663,33 @@ def test_auditlog_admin(self):
         res = self.client.get("/admin/auditlog/logentry/{}/history/".format(log_pk))
         assert res.status_code == 200
 
+
+class NoDeleteHistoryTest(TestCase):
+    def test_delete_related(self):
+        instance = SimpleModel.objects.create(integer=1)
+        assert LogEntry.objects.all().count() == 1
+        instance.integer = 2
+        instance.save()
+        assert LogEntry.objects.all().count() == 2
+
+        instance.delete()
+        entries = LogEntry.objects.order_by('id')
+
+        # The "DELETE" record is always retained
+        assert LogEntry.objects.all().count() == 1
+        assert entries.first().action == LogEntry.Action.DELETE
+
+    def test_no_delete_related(self):
+        instance = NoDeleteHistoryModel.objects.create(integer=1)
+        self.assertEqual(LogEntry.objects.all().count(), 1)
+        instance.integer = 2
+        instance.save()
+        self.assertEqual(LogEntry.objects.all().count(), 2)
+
+        instance.delete()
+        entries = LogEntry.objects.order_by('id')
+        self.assertEqual(entries.count(), 3)
+        self.assertEqual(
+            list(entries.values_list('action', flat=True)),
+            [LogEntry.Action.CREATE, LogEntry.Action.UPDATE, LogEntry.Action.DELETE]
+        )