Rails manages migration through migration files in db/migration and schema table that will be create automatically when database is created.
1. Manage through db/migration
- Each migration files is named <timestamp>_migration_name. Timestamp is the time that migration was created.
- Migration files always be sorted (ascending) by its timestamp.
2. Manage through schema_migrations table
- Each time a migration file is ran, Rails will add 1 record to schema_migrations table, the value is the timestamp of ran migration file.
- Each time we run rails db:migration Rails will check if there're migrations that has not show up in schema_migrations table, it will run those missing migrations.
- Each time we run rails db:rollback the rollback timestamp record will be deleted from schema_migrations.
