Indexes help make your database reads faster, but has the downside of making your database writes slower. Either way, you will come across the need for indexes at some point.
Mongoid has a flag called autocreate_indexes. This flag tells MongoDB to create the indexes every time a class is loaded. This is set to false by default. However, when you add an index to a model, your application will complain since there is no index in MongoDB yet.
To get around this, you can set autocreate_indexes to true. You only want to do this for development and test since this can be slow. For staging and production, you will need to log into MongoDB and create an index manually before pushing your code.