mongodb update limit更新指定数量的记录

db.user.find({status:0,country:"CN",group:"default"}).limit(50).forEach(function(doc){db.user.update({_id:doc._id},{$set:{group:"free"}})})

db.user.find({status:0,country:"CN",group:"default"}).limit(50).forEach(
    function (e) {
        e.group= "free";
        db.collection.save(e);
    }
);

user.in(_id: user.where(group: ‘default‘).limit(50).only(:_id).to_a.map{ |n| n.id }).update_all(group: ‘free‘)