Mongoid: How to load only some fields of an object you lazy load via reference?

For performance reason, I use as often as possible the `only()` keyword when writing up a mongoid query in order to specify the fields I want to load. The usual suspect, is for instance when I want a user's email of all my admins only for display purposes. I would write: ``` User.where(:groups => :admins).only(:email).each do |u| puts u.email end ``` I do this because my user model are quite full of a lot of data that I can gladly ignore when listing a bunch of emails. However, now let imagine, that my users are referenced via a Project model, so that for each project I can do: `project.user`. Thanks to mongoid's lazy loading, my object user will only get instantiated (and queried from the DB) when I call upon the reference. But what if I want to list all the email of the owner of all admin project for instance ? I would write this: ``` Project.where(:admin_type => true).each do |p| puts p.user.email end ``` The major problem here is that doing this, I load the entire user object for each projects, and if there are lots of project matching the query that could get pretty heavy. So how do I load only the emails ? I could do this: ``` User.where(:_id => p.user_id).only(:email).first.email ``` But this obviously defeat the purpose of the nice syntax of simply doing: ``` p.user.email ``` I wish I could write something like: `p.user.only(:email).email`, but I can't. Any ideas ? Alex