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
31317	Migration using CreateModel with unique_together followed by AlterUniqueTogether crashes	Adam Johnson	David Wobrock	"Full project to reproduce: https://github.com/adamchainz/django-unique-together-bug

When squashing migrations I ended up with a migration that added a model with `unique_together`, then altered the unique_together later:

{{{
migrations.CreateModel(
    name=""Book"",
    fields=[
        (
            ""id"",
            models.AutoField(
                auto_created=True,
                primary_key=True,
                serialize=False,
                verbose_name=""ID"",
            ),
        ),
        (""title"", models.CharField(default="""", max_length=100)),
    ],
    options={""unique_together"": {(""title"",)},},
),
# ...
migrations.AlterUniqueTogether(
    name=""book"", unique_together={(""title"", ""author"")},
),
}}}

This can crash when using `sqlmigrate` or `migrate` (see repo) with:

{{{
  File ""/.../django/django/db/backends/base/schema.py"", line 380, in alter_unique_together
    self._delete_composed_index(model, fields, {'unique': True}, self.sql_delete_unique)
  File ""/.../django/django/db/backends/base/schema.py"", line 414, in _delete_composed_index
    "", "".join(columns),
ValueError: Found wrong number (0) of constraints for testapp_book(title)
}}}

This is because `_delete_composed_index` from `AlterUniqueTogehter` tries to find the existing constraint name from the database.. But it hasn't been created yet, so it could never be found - its SQL is sitting in `deferred_sql`."	Bug	assigned	Migrations	dev	Normal			Ülgen Sarıkavak David Wobrock	Accepted	1	0	0	1	0	0