generate-migration

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

Deleting takes two migrations. Write both up front, but they must be two separate PRs, with phase 2 stacked on top off phase 1 so its migration depends on it. Say clearly that phase 2 can't merge until phase 1 has deployed — merging them together drops the column while old code is still running.

Phase 1 — MOVE_TO_PENDING

Run makemigrations twice, in this order. Once the field is off the model Django can't generate the AlterField anymore, so doing it the other way around means silently shipping without it.

1. With the field still on the model, edit it in place: db_constraint=False if it's an FK, null=True if it's not nullable and has no db_default. Run makemigrations to get the AlterField.
2. Remove the field and every code reference to it, then makemigrations again. Replace the generated RemoveField with SafeRemoveField(..., deletion_action=DeletionAction.MOVE_TO_PENDING) — this drops the Django state, not the column.
3. Hand-merge both into one migration. Example:

operations = [
    migrations.AlterField(
        model_name="testmodel",
        name="project",
        field=sentry.db.models.fields.foreignkey.FlexibleForeignKey(
            db_constraint=False,
            null=True,
            on_delete=django.db.models.deletion.CASCADE,
            to="sentry.project",
        ),
    ),
    SafeRemoveField(
        model_name="testmodel", name="project", deletion_action=DeletionAction.MOVE_TO_PENDING
    ),
]

Phase 2 — DELETE (second PR, merges after phase 1 deploys)

makemigrations <app> --empty, then the same SafeRemoveField with deletion_action=DeletionAction.DELETE. Nothing else in the PR.

Removing a Model (and eventually its table)

Dropping a table takes two migrations. Write both up front, but they must be two separate PRs, with phase 2 stacked on top off phase 1 so its migration depends on it. Say clearly that phase 2 can't merge until phase 1 has deployed — merging them together drops the table while old code is still running.

First, check for inbound FKs. If other tables have foreign keys pointing at this one, those columns need their own "Deleting Columns" pass, and both of its phases must be deployed before this model's phase 1 can merge.

Phase 1 — MOVE_TO_PENDING

Run makemigrations twice, in this order. Once the model is gone Django can't generate the AlterFields anymore, so doing it the other way around means silently shipping without them.

1. On each of the model's outbound FK fields, add db_constraint=False (null=True instead for a HybridCloudForeignKey), then makemigrations for the AlterField operations.
2. Remove the model and all code references, makemigrations again, and replace the generated DeleteModel with SafeDeleteModel(..., deletion_action=DeletionAction.MOVE_TO_PENDING).
3. Merge both into one migration, AlterFields first.
4. Add the table to historical_silo_assignments in src/sentry/db/router.py (or getsentry/db/router.py). Pick the silo the model used — usually SiloMode.CELL.

Dropping the constraints is not optional. The tables survive until phase 2, but Django no longer knows about them, so it can't cascade into them — a delete on a surviving parent table will fail on the leftover constraint. When removing several models at once, also drop the constraints between the pending-deletion tables, so phase 2's DROP TABLE order doesn't matter.

Phase 2 — DELETE (second PR, merges after phase 1 deploys)

makemigrations <app> --empty, then the same SafeDeleteModel with deletion_action=DeletionAction.DELETE. Leave the historical_silo_assignments entry in place — the table-drop migration needs 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.