class Human
has_one :brain
end

class Brain
belongs_to :human
end

human.brain #=> app['brains'].find({"user_id"=>{"$in"=>[BSON::ObjectId('4e1ff77971e2f4000100001e')]}})

This seems a little inefficient (especially with indexes). I'd have presumed it would have generated a query like:

app['brains'].find(

{"user_id"=>BSON::ObjectId('4e1ff77971e2f4000100001e')}

)