Agent Skills: Generate Django Database Migrations

Generate Django Database Migrations

Commands

Generate migrations automatically based on model changes:

sentry django makemigrations

For a specific app:

sentry django makemigrations <app_name>

Generate an empty migration (for data migrations or custom work):

sentry django makemigrations <app_name> --empty

After Generating

1. If you added a new model, ensure it's imported in the app's __init__.py
2. Review the generated migration for correctness
3. Run sentry django sqlmigrate <app_name> <migration_name> to verify the SQL
4. Apply the migration locally with sentry django migrate <app_name> — Sentry's migration framework runs its safety checks on apply, so this catches unsafe ops (missing is_post_deployment, unsafe column changes, etc.) before CI does.

When editing a generated migration (e.g. swapping DeleteModel for SafeDeleteModel), leave the auto-generated is_post_deployment comment block in place. It documents a non-obvious flag with concrete guidance for future migration authors — useful context, not fluff. Only remove a comment if it's stale or contradicts the code.

Don't test the ORM

Don't write tests that only exercise Django's ORM. Standard operations — create/update/delete, cascading deletes, unique-constraint enforcement — are provided by Django and Postgres and are assumed to work. Test your logic (business rules, signal receivers, custom managers/validation), not the framework's.

Do test data migrations and backfills

The exception to the above: a migration that backfills or transforms data is your logic, and it must have a test. Use the TestMigrations base class from sentry.testutils.cases; tests live in tests/sentry/migrations/.

Set app, migrate_from (the migration just before yours), and migrate_to (yours). Seed pre-migration rows in setup_before_migration(self, apps) using the historical model registry (apps.get_model("sentry", "MyModel")) — not a direct from sentry.models... import, since the current model may not match the schema at migrate_from. Then assert the post-migration state.

Write exactly one test_* method. setUp runs the full migrate-down → seed → migrate-up cycle on every test method, so each extra method pays for another round trip with no added coverage. Cover multiple cases by seeding all of them in setup_before_migration and asserting each in the single test body.

from sentry.testutils.cases import TestMigrations


class BackfillFooTest(TestMigrations):
    app = "sentry"
    migrate_from = "0123_before"
    migrate_to = "0124_backfill_foo"

    def setup_before_migration(self, apps):
        Foo = apps.get_model("sentry", "Foo")
        self.empty = Foo.objects.create(value=None)
        self.already_set = Foo.objects.create(value="kept")

    def test_backfill(self):
        self.empty.refresh_from_db()
        self.already_set.refresh_from_db()
        assert self.empty.value == "expected"
        assert self.already_set.value == "kept"

app and connection: app is the Django app label whose migration you're testing — "sentry" by default, but set it to e.g. "workflow_engine" when the migration lives in that app's migrations/ directory. connection is the database alias, "default" by default; set it to whichever connection the model's table actually lives on. Both must match where the migration and its tables actually live, or the migrate up/down will run against the wrong database.

Run these tests locally with the --migrations and --reuse-db flags. On the first run, it will be necessary to use --create-db along with --reuse-db to get the database in a good state.

Guidelines

Adding Columns

• Use db_default=<value> instead of default=<value> for columns with defaults
• Nullable columns: use null=True
• Not null columns: must have db_default set

Adding Indexes

For large tables, set is_post_deployment = True on the migration as index creation may exceed the 5s timeout.

Deleting Columns

1. Make column nullable (null=True) if not already
2. Remove all code references
3. Replace RemoveField with SafeRemoveField(..., deletion_action=DeletionAction.MOVE_TO_PENDING)
4. Deploy, then create second migration with SafeRemoveField(..., deletion_action=DeletionAction.DELETE)

Removing a Model (and eventually its table)

Two-phase process — the historical_silo_assignments entry must be added in phase 1.

Phase 1 — Remove the model class (MOVE_TO_PENDING)

1. Remove all code references
2. Replace DeleteModel with SafeDeleteModel(..., deletion_action=DeletionAction.MOVE_TO_PENDING)
3. Add the table to historical_silo_assignments in src/sentry/db/router.py (or getsentry/db/router.py for getsentry models). Pick the silo the model used — usually SiloMode.CELL.
4. Deploy

Phase 2 — Drop the table (DELETE)

After phase 1 has deployed, create a second migration with SafeDeleteModel(..., deletion_action=DeletionAction.DELETE). Leave the historical entry in place — the table-drop migration relies on it to resolve the silo.

Renaming Columns/Tables

Don't rename in Postgres. Use db_column or Meta.db_table to keep the old name.

Resolving Merge Conflicts

If migrations_lockfile.txt conflicts:

bin/update-migration <migration_name>

This renames your migration, updates dependencies, and fixes the lockfile.