Deletion on ForeignKey raises TypeError

[Markus Gerards]

Consider following model constellation:

class Catalog(models.Model):
	name = models.CharField(max_length=255)
	...

class Reader(models.Model):
	name = models.CharField(max_length=255)
	catalog = models.ForeignKey(Catalog)
	...

class ReaderHasMedia(models.Model):
	reader = models.ForeignKey(Reader)
	...

Now in some cases I need to perform following command

ReaderHasMedia.objects.filter(catalog=Catalog.objects.get(pk=123)).delete()

With this command, I get following TypeError:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 600, in delete
    deleted, _rows_count = collector.delete()
  File "/usr/local/lib/python2.7/site-packages/django/db/models/deletion.py", line 293, in delete
    deleted_counter[qs.model._meta.label] += count
TypeError: unsupported operand type(s) for +=: 'int' and 'NoneType'

With Django 1.8.7 there wasn't a count variable in deletion.py and so I believe, this is a fresh bug with Django 1.9...

[Simon Charette]

It looks like ​the fix for #16891 didn't account for the fact django.db.models.sql.subqueries.DeleteQuery.delete_qs can ​return None.

Not sure if this should be fixed at the Collector or DeleteQuery level.

[Simon Charette]

Giving it more thought I strongly believe this should be fixed in DeleteQuery.delete_qs by simply returning 0 instead of None.

[Raphael Merx]

In your example, ReaderHasMedia does not have a FK to Catalog, so ReaderHasMedia.objects.filter(catalog=Catalog.objects.get(pk=123)).delete() isn't valid.

[Markus Gerards]

Replying to raphaelmerx:

In your example, ReaderHasMedia does not have a FK to Catalog, so ReaderHasMedia.objects.filter(catalog=Catalog.objects.get(pk=123)).delete() isn't valid.

Ah! Sorry - I meant: ReaderHasMedia.objects.filter(reader__catalog=Catalog.objects.get(pk=123)).delete()

[Raphael Merx]

I can't capture the bug in a test, neither on master nor on 1.9. Am I understanding something wrong?

tests/delete_regress/models.py

@@ -139 +139 @@
 
     class Meta:
         ordering = ['name']
+
+
+class Catalog(models.Model):
+    name = models.CharField(max_length=255)
+
+class Reader(models.Model):
+    name = models.CharField(max_length=255)
+    catalog = models.ForeignKey(Catalog, models.CASCADE)
+
+class ReaderHasMedia(models.Model):
+    reader = models.ForeignKey(Reader, models.CASCADE)

tests/delete_regress/tests.py

@@ -345 +345 @@
         OrderedPerson.objects.create(name='Bob', lives_in=h)
         OrderedPerson.objects.filter(lives_in__address='Foo').delete()
         self.assertEqual(OrderedPerson.objects.count(), 0)
+
+class DoubleForeignKeyTests(TestCase):
+    def test_double_foreign_key(self):
+        from .models import Catalog, Reader, ReaderHasMedia
+        c = Catalog.objects.create(name='aaa')
+        r = Reader.objects.create(name='bbb', catalog=c)
+        ReaderHasMedia.objects.create(reader=r)
+        ReaderHasMedia.objects.filter(reader__catalog=Catalog.objects.get(pk=1)).delete()

[Simon Charette]

From looking at the stacktrace it looks like you need to run your tests against a backend that doesn't have the ​update_can_self_select feature.

The only core backend that has this feature turned off is ​MySQL.

[Simon Charette]

Created a ​PR.

@raphaelmerx thanks for the investigation. I looks like the easiest way to reproduce the issue on MySQL is the following.

class A(models.Model):
    text = models.TextField()

class B(models.Model):
    a = models.ForeignKey(A, models.CASCADE)

B.objects.delete(a__text='missing')

I suppose the reported test case didn't work because you created a Reader object. The following should do:

Reader.objects.filter(catalog__name='missing')

In order to trigger the bug you have to generate a delete query that issues a join but doesn't match any row on MySQL.

[Simon Charette <charette.s@…>]

In 8035cee9:

Fixed #25882 -- Prevented fast deletes matching no rows from crashing on MySQL.

Thanks to Trac aliases gerricom for the report, raphaelmerx for the
attempts to reproduce and Sergey Fedoseev and Tim for the review.

Refs #16891

[Simon Charette <charette.s@…>]

In c402db2e:

[1.9.x] Fixed #25882 -- Prevented fast deletes matching no rows from crashing on MySQL.

Thanks to Trac aliases gerricom for the report, raphaelmerx for the
attempts to reproduce and Sergey Fedoseev and Tim for the review.

Refs #16891

Backport of 8035cee92293f3319919c8248c7787ba43c05917 from master