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.

1 files changed, 19 insertions, 5 deletions

diff --git a/django/db/models/deletion.py b/django/db/models/deletion.py
index e3c38a03cc..dffdf808d1 100644
--- a/django/db/models/deletion.py
+++ b/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):
         """
@@ -192,11 +204,13 @@ class Collector(object):
                 field = related.field
                 if field.rel.on_delete == DO_NOTHING:
                     continue
-                sub_objs = self.related_objects(related, new_objs)
-                if self.can_fast_delete(sub_objs, from_field=field):
-                    self.fast_deletes.append(sub_objs)
-                elif sub_objs:
-                    field.rel.on_delete(self, field, sub_objs, self.using)
+                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:
+                        field.rel.on_delete(self, field, sub_objs, self.using)
             for field in model._meta.virtual_fields:
                 if hasattr(field, 'bulk_related_objects'):
                     # Its something like generic foreign key.