Fixed #16426 -- deletion of 1000+ objects with relations on SQLite

SQLite doesn't work with more than 1000 parameters in a single query.
The deletion code could generate queries that try to get related
objects for more than 1000 objects thus breaking the limit. Django now
splits the related object fetching into batches with at most 1000
parameters.

The tests and patch include some work done by Trac alias NiGhTTraX in
ticket #21205.

django/db/models/deletion.py

@@ -144,6 +144,18 @@ class Collector(object):
                 return False
         return True
 
+    def get_del_batches(self, objs, field):
+        """
+        Returns the objs in suitably sized batches for the used connection.
+        """
+        conn_batch_size = max(
+            connections[self.using].ops.bulk_batch_size([field.name], objs), 1)
+        if len(objs) > conn_batch_size:
+            return [objs[i:i + conn_batch_size]
+                    for i in range(0, len(objs), conn_batch_size)]
+        else:
+            return [objs]
+
     def collect(self, objs, source=None, nullable=False, collect_related=True,
             source_attr=None, reverse_dependency=False):
         """
@@ -192,7 +204,9 @@ class Collector(object):
                 field = related.field
                 if field.rel.on_delete == DO_NOTHING:
                     continue
-                sub_objs = self.related_objects(related, new_objs)
+                batches = self.get_del_batches(new_objs, field)
+                for batch in batches:
+                    sub_objs = self.related_objects(related, batch)
                     if self.can_fast_delete(sub_objs, from_field=field):
                         self.fast_deletes.append(sub_objs)
                     elif sub_objs:

tests/delete/tests.py

@@ -1,6 +1,9 @@
 from __future__ import unicode_literals
 
+from math import ceil
+
 from django.db import models, IntegrityError, connection
+from django.db.models.sql.constants import GET_ITERATOR_CHUNK_SIZE
 from django.test import TestCase, skipUnlessDBFeature, skipIfDBFeature
 from django.utils.six.moves import xrange
 
@@ -165,7 +168,6 @@ class DeletionTests(TestCase):
         self.assertFalse(m.m2m_through_null.exists())
 
     def test_bulk(self):
-        from django.db.models.sql.constants import GET_ITERATOR_CHUNK_SIZE
         s = S.objects.create(r=R.objects.create())
         for i in xrange(2 * GET_ITERATOR_CHUNK_SIZE):
             T.objects.create(s=s)
@@ -311,6 +313,41 @@ class DeletionTests(TestCase):
         r.delete()
         self.assertEqual(HiddenUserProfile.objects.count(), 0)
 
+    def test_large_delete(self):
+        TEST_SIZE = 2000
+        objs = [Avatar() for i in range(0, TEST_SIZE)]
+        Avatar.objects.bulk_create(objs)
+        # Calculate the number of queries needed.
+        batch_size = connection.ops.bulk_batch_size(['pk'], objs)
+        # The related fetches are done in batches.
+        batches = int(ceil(float(len(objs)) / batch_size))
+        # One query for Avatar.objects.all() and then one related fast delete for
+        # each batch.
+        fetches_to_mem = 1 + batches
+        # The Avatar objecs are going to be deleted in batches of GET_ITERATOR_CHUNK_SIZE
+        queries = fetches_to_mem + TEST_SIZE // GET_ITERATOR_CHUNK_SIZE
+        self.assertNumQueries(queries, Avatar.objects.all().delete)
+        self.assertFalse(Avatar.objects.exists())
+
+    def test_large_delete_related(self):
+        TEST_SIZE = 2000
+        s = S.objects.create(r=R.objects.create())
+        for i in xrange(TEST_SIZE):
+            T.objects.create(s=s)
+
+        batch_size = max(connection.ops.bulk_batch_size(['pk'], xrange(TEST_SIZE)), 1)
+
+        # TEST_SIZE // batch_size (select related `T` instances)
+        # + 1 (select related `U` instances)
+        # + TEST_SIZE // GET_ITERATOR_CHUNK_SIZE (delete `T` instances in batches)
+        # + 1 (delete `s`)
+        expected_num_queries = (ceil(TEST_SIZE // batch_size) +
+                                ceil(TEST_SIZE // GET_ITERATOR_CHUNK_SIZE) + 2)
+
+        self.assertNumQueries(expected_num_queries, s.delete)
+        self.assertFalse(S.objects.exists())
+        self.assertFalse(T.objects.exists())
+
 
 class FastDeleteTests(TestCase):
 
@@ -377,3 +414,15 @@ class FastDeleteTests(TestCase):
         self.assertNumQueries(2, p.delete)
         self.assertFalse(Parent.objects.exists())
         self.assertFalse(Child.objects.exists())
+
+    def test_fast_delete_large_batch(self):
+        User.objects.bulk_create(User() for i in range(0, 2000))
+        # No problems here - we aren't going to cascade, so we will fast
+        # delete the objects in a single query.
+        self.assertNumQueries(1, User.objects.all().delete)
+        a = Avatar.objects.create(desc='a')
+        User.objects.bulk_create(User(avatar=a) for i in range(0, 2000))
+        # We don't hit parameter amount limits for a, so just one query for
+        # that + fast delete of the related objs.
+        self.assertNumQueries(2, a.delete)
+        self.assertEqual(User.objects.count(), 0)