Cannot sqlmigrate on Django migrations that change unique_together

[Brіаn Lаі]

Somewhat similar to #26624, but forward sqlmigrate, reproduced on Django 3.0.8 and Postgres 10.x.

If a model has a unique_together of

unique_together = [
    (a, b)
]

and it becomes

unique_together = [
    (a, b, c)
]

, although the makemigrations command successfully generates a new migration with the corresponding AlterUniqueTogether() operation, the migration cannot be inspected with the sqlmigrate command.

Reproduce bug with the following (the project may also be attached)

class Model1(models.Model):
    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=255, unique=True)


class Model2(models.Model):
    id = models.IntegerField(primary_key=True)
    link_id = models.CharField(max_length=255)
    settings = models.ForeignKey(Model1, on_delete=models.CASCADE)


class Model3(models.Model):
    id = models.IntegerField(primary_key=True)
    order = models.IntegerField()
    model_2 = models.ForeignKey(Model2, on_delete=models.CASCADE)
    direct = models.BooleanField(default=True)

    class Meta:
        unique_together = [
            ('model_2', 'order')
            # To get the bug, change this to ('model_2', 'order', 'direct') and sqlmigrate the migration that this creates.
        ]

The error shown in the terminal is:

Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/lib/python3.6/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/lib/python3.6/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/lib/python3.6/site-packages/django/core/management/base.py", line 328, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/lib/python3.6/site-packages/django/core/management/commands/sqlmigrate.py", line 30, in execute
    return super().execute(*args, **options)
  File "/lib/python3.6/site-packages/django/core/management/base.py", line 369, in execute
    output = self.handle(*args, **options)
  File "/lib/python3.6/site-packages/django/core/management/commands/sqlmigrate.py", line 65, in handle
    sql_statements = executor.collect_sql(plan)
  File "/lib/python3.6/site-packages/django/db/migrations/executor.py", line 225, in collect_sql
    state = migration.apply(state, schema_editor, collect_sql=True)
  File "/lib/python3.6/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/lib/python3.6/site-packages/django/db/migrations/operations/models.py", line 525, in database_forwards
    getattr(new_model._meta, self.option_name, set()),
  File "/lib/python3.6/site-packages/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 "/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 414, in _delete_composed_index
    ", ".join(columns),
ValueError: Found wrong number (0) of constraints for bar_model3(model_2, order)

[Brіаn Lаі]

Bug demo project