I suck at mongoid

1_user.rb

class User
  include Mongoid::Document
  references_one :best_friend, :class_name => "Person"
  references_one :worst_enemy, :class_name => "Person"
end

2_person.rb

class Person
  include Mongoid::Document
end

3_i_fail.rb

person = Person.create!
user = User.create! :best_friend => person
user.best_friend  # => #<Person _id: 4d781dbd568ab57112000002, _type: nil, _id: BSON::ObjectId('4d781dbd568ab57112000002')>
user.reload
user.best_friend # => nil

Your Person model is missing the inverse reference.
class Person
include Mongoid::Document
referenced_in :user
end

I'm not sure how the custom naming will fit in, though.