Mongoid: belongs_to user and has_one user

I'm using Mongoid for my app and I have a problem setting up correct relationships for Users and subscriptions.

All I need to do is to make a simple "has one and belongs to one" relationship for UserSubscription model.

class User
  has_many :user_subscriptions
end

class UserSubscription
  belongs_to :user

  has_one :user # user2 to which user1 is subscribed
  field :category, :String
end

All I want to do is to have a list of subscriptions for each user:

> user1.user_subscriptions # list of subscription objects
> user1.user_subscriptions << UserSubscription.create(:user => user2, :category => 'Test')
> user1.user_subscriptions.where(:user => user2).delete_all

How to implement this? Thank you for help.