saleor-migrations

name	saleor-migrations
description	Rules for writing safe Django migrations in Saleor that don't lock tables or break zero-downtime deploys. Use whenever creating or editing a migration (schema or data), including field removals and index/constraint changes.

Writing Saleor migrations

Saleor deploys with zero downtime across many pods against a shared Postgres. A migration that takes a long table lock stalls every pod. Follow these rules.

Keep locks short: split migrations

• One model per migration, and one field change per migration. Each schema operation holds a lock on the table for its duration; batching several into one migration multiplies the locked time.
• The only valid reason to combine is separating a schema migration from its data migration.
• Always separate schema changes and data migration.
• Don't add a migration that re-alters a column an earlier migration already changed — check the existing migration history first.
• Give migrations descriptive names that reflect the actual operation (0070_add_payment_gift_card_brand, not 0070_alter_payment_partial_add_db_default for something else).

Indexes and constraints: create concurrently

• Add unique constraints/indexes concurrently using the established non-blocking pattern (see e.g. page migration 0030_slug_translation_unique_constraint) — a plain AddConstraint / AddIndex takes a blocking ACCESS EXCLUSIVE lock and can stall writes across all pods.
• Enforce value invariants (e.g. non-negative balance) with a DB CheckConstraint, not just application logic.

Backwards compatibility: the new schema must work with the old code

During a rolling deploy, pods running the previous minor version talk to the already-migrated database. Django SELECTs and UPDATEs every column it knows about, so the schema must stay valid for that old ORM. Making the DB backwards-compatible is the default; changing old code requires crafting two releases at once — reserve it for cases where nothing else works.

Adding a field

Old pods insert rows without knowing the column, so it must be writable without them: null=True or a db_default. Plain Django default= is not enough — it lives in Python and never reaches the database.

Removing a field: stage it across three releases

Removing a NOT NULL / defaulted column in one step can fail mid-deploy while old and new pods coexist.

1. N — add a db_default (or null=True) so the DB can write the column without the ORM.
2. N+1 — de-register the field from the ORM, leaving the column in place, via SeparateDatabaseAndState: state_operations=[RemoveField(...)] and database_operations that make the column nullable. Old pods still find the column; new pods no longer touch it.
3. N+2 — drop the column, now that no process uses it. Track it as an explicit follow-up.

migrations.SeparateDatabaseAndState(
    database_operations=[
        migrations.AlterField(
            model_name="sitesettings",
            name="automatically_confirm_all_new_orders",
            field=models.BooleanField(null=True, blank=True),
        ),
    ],
    state_operations=[
        migrations.RemoveField(
            model_name="sitesettings",
            name="automatically_confirm_all_new_orders",
        ),
    ],
)

Keep any legacy enum values / code retained only for migration safety tracked as a removal task with a "remove in X.Y" note.

Renaming or moving a field

Avoid unless necessary — a rename is an add plus a remove, so it costs the same three releases.

1. N — add the new field (null=True), and write both old and new fields everywhere the old one is written, so old pods keep seeing valid data. Add a data migration that backfills the new field. Note in the upgrade guide that N+1 requires upgrading to this patch release first.
2. N+1 — read from the new field. Re-run the backfill data migration (old pods may have inserted rows in the old format between step 1's migration and the cutover), then drop null=True from the new field. De-register the old field per "Removing a field" step 2.
3. N+2 — drop the old column.

Handle "new field is null but old one isn't" while both exist.

Any data migration that reshapes data written by old pods runs twice — once before the new code deploys, once in the next version when the old code is provably gone.

Data migrations

• A data migration must be all-or-nothing: process everything or nothing. Don't abort partway on a fixed depth/count cap and leave a partial migration.
• post_migrate sender must be the migration's own app config — a common copy-paste bug is registry.get_app_config("product") inside an account/order migration.
• Use a module/task constant (like BATCH_SIZE) for internal tuning knobs, not an env var nobody will set.
• When cleaning up (e.g. removing a permission), address all models that hold the value (App, AppExtension, AppInstallation, …), or document why one is handled elsewhere.
• Watch for per-iteration DB queries (O(N) vs O(1)); batch related lookups.

Cross-branch ports

• Keep a ported migration's filename identical to its counterpart on the other branch, and add a merge migration where histories diverge.
• Keep a ported migration's filename identical to its counterpart on the other branch, and add a merge migration where histories diverge using ./manage.py makemigrations --merge.

Before requesting review

• Confirm each migration touches a single model and a single field, and that any index or constraint is created concurrently rather than with a blocking operation.
• Confirm every destructive column change is staged across releases (add db_default, then remove the field from the ORM via SeparateDatabaseAndState, then drop the column in a later version).
• Confirm every new column is nullable or has a db_default — a Python-only default= leaves old pods unable to insert.
• Confirm any data migration over data old pods still write is scheduled to run again in the next version.
• Confirm each post_migrate handler passes its own app config as the sender, and that every data migration is all-or-nothing rather than aborting partway.
• Run the migration locally with manage.py migrate and confirm it applies cleanly.