LightMongo-Rails provides ActionPack compatibility for LightMongo, allowing you to use form_for and other such helpers.
This project is long abandoned in favour of other contemporary ORMs.
gem install light_mongo-rails, assuming you already have LightMongo and Rails 3 on the boil.
class MyModel
include LightMongo::Model
end
Then use like a normal LightMongo::Document.
NB: You are still welcome to include LightMongo::Document when in Rails, you just might hit some trouble using the ActionPack helpers.