Mongoid 2.x med 10gens driver:
BSON::ObjectId.new('506144650ed4c08d84000001')
Mongoid 3 med knallert:
Moped::BSON::ObjectId.from_string('506144650ed4c08d84000001')
Mongoid 4 (knallert) / Mongoid 5/6/7 (mongo):
BSON::ObjectId.from_string('506144650ed4c08d84000001')