Single Collection Inheritance (“SCI”) allows you to store multiple types of similar documents into a single collection. The most common case is when you have a hierarchy of objects that inherit behavior and attributes.
When you inherit from a model, MongoMapper adds at
_type attribute to your model, which is set when your model is created.
Why I Think Mongo is to Databases What Rails was to Frameworks see 2. Single Collection Inheritance Gone Wild
Single Table Inheritance in Rails by Alex Reisner. A good resouce on the why, what, and when of STI.