For MongoDB you don't need migrations, because it's schema less. Just store a record and it will appear in your database.
And because it IS schema less, you can change your Rails model, without migrating your database, MongoDB won't bother. But be sure, that your application can handle both versions of your stored stuff.
But if you want, you CAN write migrations, that transform your old data, into your new schema, when you have changed something in your Rails models (but I would start writing migrations not before I have some production data, for development it's ok to drop the DB and recreate it). But as I said, that only bothers your Rails application. MongoDB allows you to do everything without migrations.