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
28299	Adding a unique UUIDField to an existing model immediately fails with a database constraint error	Tom Forbes	nobody	"If you want to add a unique UUIDField to an existing model, you would add the following field definition (as per the docs):

`column = models.UUIDField(unique=True, default=uuid.uuid4)`

However, upon creating and running a migration Django calls the `default` function *once*, then attempts to update all rows in a single query, which obviously violates the unique constraint and throws a database error.

This is particularly annoying and possibly confusing to new users (the error message is not exactly descriptive), and there does not seem to be an easy way of overriding it without creating your own custom RunPython migrations that sequentially calls the default function for each row.

Because it's pretty common to have a unique `UUIDField`, and because it's a problem you immediately run into when adding it to an existing model, could a workaround be added in the migrations framework to be a bit smarter about this? Or perhaps in a more generic sense, if `unique=True` then re-evaluate the `default` for each row rather than once during an `UPDATE` statement."	New feature	closed	Migrations	dev	Normal	duplicate			Unreviewed	0	0	0	0	0	0