Fixed #15579 -- Added ability to delete only child models in multi-table inheritance.

django/db/models/base.py

@@ -827,7 +827,7 @@ class Model(six.with_metaclass(ModelBase)):
         return manager._insert([self], fields=fields, return_id=update_pk,
                                using=using, raw=raw)
 
-    def delete(self, using=None):
+    def delete(self, using=None, keep_parents=False):
         using = using or router.db_for_write(self.__class__, instance=self)
         assert self._get_pk_val() is not None, (
             "%s object can't be deleted because its %s attribute is set to None." %
@@ -835,7 +835,7 @@ class Model(six.with_metaclass(ModelBase)):
         )
 
         collector = Collector(using=using)
-        collector.collect([self])
+        collector.collect([self], keep_parents=keep_parents)
         collector.delete()
 
     delete.alters_data = True

django/db/models/deletion.py

@@ -174,7 +174,7 @@ class Collector(object):
             return [objs]
 
     def collect(self, objs, source=None, nullable=False, collect_related=True,
-            source_attr=None, reverse_dependency=False):
+            source_attr=None, reverse_dependency=False, keep_parents=False):
         """
         Adds 'objs' to the collection of objects to be deleted as well as all
         parent instances.  'objs' must be a homogeneous iterable collection of
@@ -189,6 +189,9 @@ class Collector(object):
         current model, rather than after. (Needed for cascading to parent
         models, the one case in which the cascade follows the forwards
         direction of an FK rather than the reverse direction.)
+
+        If 'keep_parents' is False, data of parent's models will be not
+        deleted.
         """
         if self.can_fast_delete(objs):
             self.fast_deletes.append(objs)
@@ -200,6 +203,7 @@ class Collector(object):
 
         model = new_objs[0].__class__
 
+        if not keep_parents:
             # Recursively collect concrete model's parent models, but not their
             # related objects. These will be found by meta.get_fields()
             concrete_model = model._meta.concrete_model

docs/ref/models/instances.txt

@@ -533,7 +533,7 @@ value, the field will be added to the updated fields.
 Deleting objects
 ================
 
-.. method:: Model.delete([using=DEFAULT_DB_ALIAS])
+.. method:: Model.delete([using=DEFAULT_DB_ALIAS, keep_parents=False])
 
 Issues an SQL ``DELETE`` for the object. This only deletes the object in the
 database; the Python instance will still exist and will still have data in
@@ -545,6 +545,14 @@ For more details, including how to delete objects in bulk, see
 If you want customized deletion behavior, you can override the ``delete()``
 method. See :ref:`overriding-model-methods` for more details.
 
+Sometimes with :ref:`multi-table inheritance <multi-table-inheritance>` you may
+want to delete only a child model's data. Specifying ``keep_parents=True`` will
+keep the parent model's data.
+
+.. versionchanged:: 1.9
+
+    The ``keep_parents`` parameter was added.
+
 Pickling objects
 ================
 

docs/releases/1.9.txt

@@ -151,6 +151,10 @@ Models
   managers created by ``ForeignKey``, ``GenericForeignKey``, and
   ``ManyToManyField``.
 
+* Added the ``keep_parents`` parameter to :meth:`Model.delete()
+  <django.db.models.Model.delete>` to allow deleting only a child's data in a
+  model that uses multi-table inheritance.
+
 CSRF
 ^^^^
 

tests/delete/tests.py

@@ -349,6 +349,13 @@ class DeletionTests(TestCase):
         self.assertFalse(S.objects.exists())
         self.assertFalse(T.objects.exists())
 
+    def test_delete_with_keeping_parents(self):
+        child = RChild.objects.create()
+        parent_id = child.r_ptr_id
+        child.delete(keep_parents=True)
+        self.assertFalse(RChild.objects.filter(id=child.id).exists())
+        self.assertTrue(R.objects.filter(id=parent_id).exists())
+
 
 class FastDeleteTests(TestCase):