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