Beware changing the "related name" of a Django model field

Posted by David Winterbottom Head of Engineering on Mar 18, 2019

We had an outage on Friday 15th March caused by an innocent-looking change, where the “related name” of a Django model field was renamed.

We had wrongly assumed this didn’t require any schema changes to our Postgres 9.5 database, but it did: it dropped and recreated the foreign-key constraints on the table.

You can reproduce this behaviour by running sqlmigrate on a migration created after renaming a related_name attribute:

$ ./manage.py sqlmigrate $APP_LABEL $MIGRATION_NUMBER

which yields something like:

BEGIN;
--
-- Alter field foo on bar
--
SET CONSTRAINTS "bar_foo_id_xxx_fk" IMMEDIATE; 
ALTER TABLE "bar" DROP CONSTRAINT "bar_foo_id_xxx_fk";
ALTER TABLE "bar" ADD CONSTRAINT "bar_foo_id_xxx_fk" 
FOREIGN KEY ("foo_id") REFERENCES "foo" ("id") 
DEFERRABLE INITIALLY DEFERRED;
COMMIT;

Adding a foreign-key constraint requires an ACCESS EXCLUSIVE lock on the table, blocking SELECT queries. For large tables, adding the constraint may take a while, which can lead to major operational problems as queries queue up. This is what happened to us.

FYI, this behaviour in Django is inadvertent: there is an open bug in Django 2.1.x and below: #25253,

There isn’t an good workaround for this problem. The best approach we know of is to update an already-applied migration file to reflect the new related name. Since such a migration is already applied, no SQL will be run against your schema when this change deploys and Django won’t pick up any changes when makemigrations is run.

During our next internal blitzday (where we work on our tooling, dependencies and general codebase health), we’ll see if we can submit a patch to Django to fix this, to avoid anyone else getting caught out like we did.