Есть другое решение. Если между классами Person и Preference существует отношение «многие ко многим», то:
ruby-1.9.2-p0 > Person.count
=> 0
ruby-1.9.2-p0 > Preference.count
=> 0
ruby-1.9.2-p0 > person = Person.create
=> #< Person _id: 4cd353e92b58af214b000006, preference_ids: []>
ruby-1.9.2-p0 > pref = Preference.create
=> #< Preference _id: 4cd353ee2b58af214b000007, person_ids: [], name: nil>
ruby-1.9.2-p0 >
ruby-1.9.2-p0 > person.preferences << pref
=> true
ruby-1.9.2-p0 > Preference.first.people.count
=> 1
ruby-1.9.2-p0 > Person.first.preferences.count
=> 1
ruby-1.9.2-p0 >
ruby-1.9.2-p0 > person.preferences.first.name = 'foobar'
=> "foobar"
ruby-1.9.2-p0 > person.preferences.first.save
=> true
ruby-1.9.2-p0 > pref.reload
=> #< Preference _id: 4cd353ee2b58af214b000007, person_ids: [BSON::ObjectId('4cd353e92b58af214b000006')], name: "foobar">
ruby-1.9.2-p0 > pref.name
=> "foobar"