Rerun stale migration in Rails

Consider the following scenario. You are reviewing a pull request (PR) of your colleague. That PR has a migration that adds a new table into the DB. You run that migration to test the PR. The migration creates settings table. Forgetting to reverse the migration so that it doesn’t mess up your local DB state, you go to another branch and continue your work. Later, the PR gets this table renamed to user_settings. Finally, it gets merged.

Then you update the main branch and run migrations. Surprisingly, you’ve got changes in schema.rb. Git shows that it wants to add settings and remove user_settings table. You realize that you’ve run the migration that’s kinda stale now. You try to roll this migration back with rails db:migrate:down VERSION=20220505072316 command. No luck:

ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  table "user_settings" does not exist
: DROP TABLE "user_settings"

Dead-end? See how to cope with it.

Solution

To understand which migration is run and which is not Rails has a special table in DB schema_migrations. Look at its structure inside DB console (jump into it with rails db command or use any other DB client). The is the description of this table inside PostgreSQL:

~# \d schema_migrations
                 Table "public.schema_migrations"
 Column            Type           Collation  Nullable  Default
═════════╪════════════════════════╪═══════════╪══════════╪═════════
 version  character varying(255)             not null 
Indexes:
    "unique_schema_migrations" UNIQUE, btree (version)

Whenever you run migration it inserts a new row into this table with column version equal to the timestamp of the migration file. For example, a migration inside file 20220505072316_create_settings.rb inserts row with version=20220505072316.

Next time when you run migrations Rails checks if that migration file timestamp is already inside schema_migrations table. If it’s not there, it applies the migration, otherwise skips it. That way rails db:migrate command guarantees idempotency.

That knowledge gives us a glue on what to do. If delete that migration timestamp from schema_migrations we can run it again. That will add user_settings table but won’t delete the stale settings table. But that’s not a big deal - we can do that manually in the DB console. Once we do that rails db:migrate command won’t generate any diff anymore.

Run these commands within the DB console:

delete from schema_migrations where version = '20220505072316';
drop table settings;

Run rails db:migrate and make sure there are no changes on your schema.rb.