class Entry
  include Mongoid::Document
  field :content, type: String
 
  has_and_belongs_to_many :favorite_of, class_name: 'User', inverse_of: :favorites
  belongs_to :user, inverse_of: :entries, class_name: 'User'
end
 
class User
  include Mongoid::Document
  field :name, type: String
 
  has_and_belongs_to_many :favorites, class_name: 'Entry', inverse_of: :favorite_of
  has_many :entries, inverse_of: :user, class_name: 'Entry'
end

irb(main):001:0> u = User.create
=> #<User _id: 51223cd9e44f7b5ce2000001, name: nil, favorite_ids: nil>
irb(main):002:0> e = Entry.create
=> #<Entry _id: 51223cdde44f7b5ce2000002, content: nil, favorite_of_ids: nil, user_id: nil>
irb(main):003:0> u.favorites << e
=> [#<Entry _id: 51223cdde44f7b5ce2000002, content: nil, favorite_of_ids: ["51223cd9e44f7b5ce2000001"], user_id: nil>]
irb(main):004:0> u.save
=> true
irb(main):005:0> u.favorites
=> [#<Entry _id: 51223cdde44f7b5ce2000002, content: nil, favorite_of_ids: ["51223cd9e44f7b5ce2000001"], user_id: nil>]
irb(main):006:0> exit


$ rails c              
Loading development environment (Rails 3.2.12)
irb(main):001:0> User.first.favorites
=> []