Add Rollback A Couple Migrations as a Rails TIL

rails/rollback-a-couple-migrations.md

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+# Rollback A Couple Migrations
+
+Let's say we need to rollback a couple Rails migrations that have been applied
+to our local environment. We run `rails db:migrate:status` and see that there
+are _2_ migrations that we want to _undo_.
+
+We can accomplish this by using the `STEP` env var with the rollback command.
+
+```bash
+$ rails db:rollback STEP=2
+```
+
+Just set `STEP` to the number of migrations that we need to rollback. If we
+then rerun `rails db:migrate:status` we'll now see those latest two migrations
+are `down`.
+
+Note: by default Rails doesn't like to operate with pending migrations. If we
+want to temporarily disable the pending migration check, we can alter the
+migration error config in `config/development.rb`.
+
+```diff
+  # Raise an error on page load if there are pending migrations.
+-  # config.active_record.migration_error = :page_load
++  config.active_record.migration_error = false
+```