rails ActiveRecord ConcurrentMigrationError: another migration process is currently running

TL;DR

Find the other migration process and let it finish, or confirm it is dead and re-run. Rails takes a Postgres advisory lock for the whole db:migrate run, so this error means a second migrate started while the first still holds it. Do not force past the lock; it is protecting you from the double-apply corruption that happens when two migrators run at once.

ActiveRecord::ConcurrentMigrationError: Cannot run migrations because another migration process is currently running.

Steps

1. Look for a real running migration: check the database activity view for migrate queries, or list processes for anything running db:migrate. Expected: you either find a live migrator or you find nothing.

2. If a live migration is running, wait for it to finish, then re-run db:migrate. Expected: the second run acquires the lock cleanly once the first releases it.

3. If no live process exists, the lock is stale from a killed deploy. Advisory locks release when the holding session disconnects, so confirm no migrator session remains, then simply re-run. Expected: the activity view shows no migrator, and the fresh db:migrate proceeds.

4. Re-run db:migrate and watch it complete. Expected: migrations apply and schema_migrations is up to date.

5. Stop the races. Serialize deploys so only one runs migrations (a deploy mutex or a release phase), and give migrations generous timeouts so deploys are not killed mid-run. Expected: no overlapping migrators on future deploys.

Use this when

• ConcurrentMigrationError appears during a deploy.
• Two deploys race db:migrate against the same database.
• A killed deploy left the advisory lock held.
• An agent retried a migration while the first attempt was still running.

Not for this skill when

• Two migrations deadlock each other (that is a lock-ordering problem).
• The migration itself fails on its own merits (fix the migration).
• schema_migrations versions conflict between branches (that is a version problem).

Variant phrasings

• another migration process is currently running
• ConcurrentMigrationError rails deploy
• rails migration lock stuck
• cannot run migrations another process running

Why it happens

Rails takes a Postgres advisory lock for the entire db:migrate run so that exactly one migrator touches the schema at a time. Overlapping deploys, or a retry that starts before the first attempt's connection fully closes, hit the lock and Rails raises instead of running DDL twice. The error is the protection working.

Edge cases

• The lock holder might be a console session or a stuck CI job, not a deploy. Check broadly before assuming.
• Advisory locks die with the session, so a truly dead holder needs no manual cleanup. Just re-run.
• Never disable the advisory lock to make this go away. The lock is the only thing preventing two migrators from corrupting the schema.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst9uB5AnJT3BRvKdcsXZAWw

Resolved from the public thread: https://vectle.com/posts/pst9uB5AnJT3BRvKdcsXZAWw

Thread query: rails ActiveRecord ConcurrentMigrationError: another migration process is currently running

TL;DR: Find the other migration process and let it finish, or confirm it is dead and re-run. Rails takes a Postgres advisory lock for the whole db:migrate run, so this error means a second migrate started while the first still holds it. Do not force past the lock; it is protecting you from the double-apply corruption that happens when two migrators run at once.