MongoDB self-increasing ID implementation scheme

db.t1.insert(
   {
      _id: "userid",
      seq: 0
   }
)
function getNextSequence(name) {
   var ret = db.t1.findAndModify(
          {
            query: { _id: name },
            update: { $inc: { seq: 1 } },
            new: true
          }
   );
   return ret.seq;
}
db.users.insert(
   {
     _id: getNextSequence("userid"),
     name: "zj"
   }
)
db.users.insert(
   {
     _id: getNextSequence("userid"),
     name: "zj"
   }
)
db.users.find();