Single Collection Inheritance

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.

class Field
  include MongoMapper::Document
  key :name, :required => true
end

class FileUpload < Field
  plugin Joint
  attachment :file
  validates_length_of :file_size, :minimum => 0, :maximum => 10.megabytes
end

class TextField < Field
end

class RadioButton < Field
  many :options
end

When you inherit from a model, MongoMapper adds at _type attribute to your model, which is set when your model is created.

Extra Resources

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.