*scratch*.rb

class Ship
  include Mongoid::Document
  has_one :vehicle
end
class Vehicle
  include Mongoid::Document
  belongs_to :ship
end

irb(main):009:0> s = Ship.new
=> #<Ship _id: 523a43d3d291624cb8000001, >
irb(main):010:0> s.vehicle = Vehicle.new
=> #<Vehicle _id: 523a43dbd291624cb8000002, ship_id: "523a43d3d291624cb8000001">
irb(main):011:0> s.save!
  MOPED: 127.0.0.1:27017 COMMAND      database=admin command={:ismaster=>1} (11.9979ms)
  MOPED: 127.0.0.1:27017 INSERT       database=echo_dev collection=ships documents=[{"_id"=>"523a43d3d291624cb8000001"}] flags=[] (0.1149ms)
=> true
irb(main):012:0> Vehicle.count
  MOPED: 127.0.0.1:27017 COMMAND      database=echo_dev command={:count=>"vehicles", :query=>{}} (7.9939ms)
=> 0