I'm building a Rails application using MongoDB as the back-end and MongoMapper as the ORM tool. Suppose in version 1, I define the following model:

class SomeModel
  include MongoMapper::Document
  key :some_key, String
end

Later in version 2, I realize that I need a new required key on the model. So, in version 2, SomeModel now looks like this:

class SomeModel
  include MongoMapper::Document
  key :some_key, String
  key :some_new_key, String, :required => true
end

How do I migrate all my existing data to include some_new_key? Assume that I know how to set a reasonable default value for all the existing documents. Taking this a step further, suppose that in version 3, I realize that I really don't need some_key at all. So, now the model looks like this

class SomeModel
  include MongoMapper::Document
  key :some_new_key, String, :required => true
end

But all the existing records in my database have values set for some_key, and it's just wasting space at this point. How do I reclaim that space?

With ActiveRecord, I would have just created migrations to add the initial values of some_new_key (in the version1 -> version2 migration) and to delete the values for some_key (in the version2 -> version3 migration).

What's the appropriate way to do this with MongoDB/MongoMapper? It seems to me that some method of tracking which migrations have been run is still necessary. Does such a thing exist?

EDITED: I think people are missing the point of my question. There are times where you want to be able to run a script on a database to change or restructure the data in it. I gave two examples above, one where a new required key was added and one where a key can be removed and space can be reclaimed. How do you manage running these scripts? ActiveRecord migrations give you an easy way to run these scripts and to determine what scripts have already been run and what scripts have not been run. I can obviously write a Mongo script that does any update on the database, but what I'm looking for is a framework like migrations that lets me track which upgrade scripts have already been run.