I add the eager loading feature for mongoid, just like ActiveRecord3, so you can use such syntax <code>Post.limit(20).includes(:person).each
{ |post| puts post.person.name }</code>.
ActiveRecord3 has two options for eager loading, one is JOIN, the other is to use two sql queries. As mongodb doesn't support JOIN, so I just implement eager loading as two queries for mongoid.
I also did a benchmark for eager loading, about 20% improvement, http://gist.github.com/619142