title: Managing mongoid migrations
body: <p>Can someone give me a short introduction to doing DB migrations in Rails using Mongoid? I'm particularly interested in lazy per document migrations. By this, I mean that whenever you read a document from the database, you migrate it to its latest version and save it again. </p>
  
  <p>Has anyone done this sort of thing before? I've come across <a href="https://github.com/adacosta/mongoid_rails_migrations">mongoid_rails_migrations</a>, but it doesn't provide any sort of documentation, and although it looks like it does this, I'm not really sure how to use it.</p>
  
  <p>I should point out I'm only conceptually familiar with ActiveRecord migrations.</p>