Squashing migrations from unique=True to unique=False to UniqueConstraint produces irreversible migration

[Jacob Walls]

Rhymes a bit with #31503, just in the reverse direction.

• Create a model with a unique=True field, create a migration. I used URLField.
• Create an empty migration, e.g with migrations.RunSQL(sql="SELECT 1", reverse_sql=""). (This will prevent the next AlterField from optimizing out when squashing. There are likely other possible reproducers without this step.)
• Alter the field from step 1 to have unique=False, create a migration
• Add a UniqueConstraint to the model that involves just that field, create a migration
• Squash the four migrations
• Migrate forward
• Migrate to zero, with or without removing the other migrations or the replaced attribute

Result:

  Unapplying polls.0001_initial_squashed_0004_menu_unique_site...Traceback (most recent call last):
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/utils.py", line 87, in _execute
    return self.cursor.execute(sql)
           ^^^^^^^^^^^^^^^^^^^^^^^^
psycopg2.errors.DuplicateTable: relation "polls_menu_site_61d71486_like" already exists


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/jwalls/prj/night/manage.py", line 22, in <module>
    main()
  File "/Users/jwalls/prj/night/manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/core/management/base.py", line 412, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/core/management/base.py", line 458, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/core/management/base.py", line 106, in wrapper
    res = handle_func(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/core/management/commands/migrate.py", line 356, in handle
    post_migrate_state = executor.migrate(
                         ^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/migrations/executor.py", line 141, in migrate
    state = self._migrate_all_backwards(plan, full_plan, fake=fake)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/migrations/executor.py", line 219, in _migrate_all_backwards
    self.unapply_migration(states[migration], migration, fake=fake)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/migrations/executor.py", line 279, in unapply_migration
    state = migration.unapply(state, schema_editor)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/migrations/migration.py", line 193, in unapply
    operation.database_backwards(
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/migrations/operations/fields.py", line 240, in database_backwards
    self.database_forwards(app_label, schema_editor, from_state, to_state)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/migrations/operations/fields.py", line 235, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/base/schema.py", line 831, in alter_field
    self._alter_field(
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/postgresql/schema.py", line 304, in _alter_field
    self.execute(like_index_statement)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/postgresql/schema.py", line 48, in execute
    return super().execute(sql, None)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/base/schema.py", line 201, in execute
    cursor.execute(sql, params)
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/utils.py", line 102, in execute
    return super().execute(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/utils.py", line 67, in execute
    return self._execute_with_wrappers(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/utils.py", line 80, in _execute_with_wrappers
    return executor(sql, params, many, context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/utils.py", line 84, in _execute
    with self.db.wrap_database_errors:
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/utils.py", line 91, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/Users/jwalls/release/lib/python3.12/site-packages/django/db/backends/utils.py", line 87, in _execute
    return self.cursor.execute(sql)
           ^^^^^^^^^^^^^^^^^^^^^^^^
django.db.utils.ProgrammingError: relation "polls_menu_site_61d71486_like" already exists

*
failing squashed migration:

# Generated by Django 4.2.13 on 2024-05-21 00:59

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = []

    operations = [
        migrations.CreateModel(
            name="Menu",
            fields=[
                (
                    "id",
                    models.BigAutoField(
                        auto_created=True,
                        primary_key=True,
                        serialize=False,
                        verbose_name="ID",
                    ),
                ),
                ("site", models.URLField(unique=True)),
            ],
        ),
        migrations.RunSQL(
            sql="SELECT 1",
            reverse_sql="",
        ),
        migrations.AlterField(
            model_name="menu",
            name="site",
            field=models.URLField(),
        ),
        migrations.AddConstraint(
            model_name="menu",
            constraint=models.UniqueConstraint(fields=("site",), name="unique_site"),
        ),
    ]

My final model looked like:

from django.db import models

class Menu(models.Model):
    site = models.URLField()

    class Meta:
        constraints = [
            models.UniqueConstraint(fields=["site"], name="unique_site")
        ]

Tested on postgres 14.3.2