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
25614	Changing ForeignKey(on_delete=...) unnecessarily drops and recreates constraints	Jeremy Dunck	nobody	"Recently I changed the on_delete attributes of many ForeignKeys on a MySQL database.

This resulted in an auto-detected migration that drops and recreates the constraint for each FK, despite the effective constraint remaining the same (e.g.  FOREIGN KEY (`modified_by_id`) REFERENCES `common_user` (`id`); )

The [https://github.com/django/django/blob/1.8.5/django/db/migrations/autodetector.py#L848 autodetection] just compares the deconstruction.

The [https://github.com/django/django/blob/1.8.5/django/db/backends/base/schema.py#L486 schema editor] drops the FK and recreates it.

It's not entirely clear to me which part needs to be changed - whether the deconstruction should not include on_delete, or whether the autodetector should discard on_delete from consideration, or whether the editor should check for parameters that actually change the effective FK.

What is clear is that dropping and recreating FKs isn't a fast operation on large databases, and it would be good to avoid where possible."	Bug	closed	Migrations	1.8	Normal	duplicate			Unreviewed	0	0	0	0	0	0