Hi, this doesn't seem right:

#!/usr/bin/env ruby

require 'mongoid'

Mongoid.configure do |config|
config.connect_to "mongoid_test"
end

class A
include Mongoid::Document

field :a, type: String
field :b, type: String
end

class B
include Mongoid::Document

field :c, type: String, default: 'c has been initialized'
end

puts B.new.inspect

A.delete_all
A.create(a: 'a', b: 'b')

A.only(:a).each do |a|
puts B.new.inspect
end

Output:

$./test_only.rb
#<B _id: 5048f28f66d9f1e55a000001, _type: nil, c: "c has been initialized">
#<B _id: , _type: nil, c: nil>

I've been bitten by this again and again. I don't think anyone would expect the call to A.only() to affect B.new executed inside that block. Or any other mongoid method called on anything except maybe the yielded a object.