id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
30023	SQLite schema editor can cause table corruption if unsed within a transaction since Django 2.0	Simon Charette	Simon Charette	"

From Django 2.0+ SQLite schema editor requires foreign key constraint checking to be disabled before performing any operations to make sure foreign keys are not left pointing at `__old` tables when performing operations requiring table rebuilds.

Since [https://sqlite.org/foreignkeys.html#fk_enable SQLite prevents foreign key constraint checking from being disabled within a transaction] `SchemaEditor(atomic=True).__enter__()` has no choice but to disable foreign key constraints **before** opening the transaction meant to ensure atomic DDL operations.

One edge case that `SchemaEditor` doesn't account for though is that the it might contextually be used within an already opened transaction that would prevent foreign key constraints from being effectively disabled and result in silent referent table corruption when an operation requiring a table is rebuild id performed.

In order to prevent this from happening `SchemaEditor().__enter__()` should ensure foreign key constraint checks are effectively disabled after requesting it and error out if it's not the case.

This assertion should be more adequate than preventing schema editor from being used in a transaction altogether as disabling constraint checks before opening a transaction works just fine as well.

For example

{{{#!python
with transaction.atomic():
    call_command('migrate')
}}}

Just has to be converted to

{{{#!python
with connection.constraint_checks_disabled(), transaction.atomic():
    call_command('migrate')
}}}

And work just fine.

''This is was originally reported in #29182 but was hijacked to deal with a SQLite 3.26 issue with similar symptoms and can be reproduced in a [https://github.com/ezaquarii/django-sqlite-migration-bug test project]."	Bug	assigned	Migrations	2.0	Release blocker			ezaquarii	Accepted	0	0	0	0	0	0