I am using [Mongo Mapper](http://mongomapper.com/) and I'm trying to figure out how you make a document required. For example, I want to do something like this:

```
class Question
  include MongoMapper::Document

  many :answers
end

class Answer
  include MongoMapper::Document

  belongs_to :question, :required => true
end

```

But when I do, it's doesn't enforce that the answer have a question:

```
> Answer.new.save
 => true

```