Simple create auto increment mongo

Auto_increment

// Adding counter
db.counters.insert(
   {
      _id: "userid",
      seq: 0
   }
);

// function to add sequence
function getNextSequence(name) {
   var ret = db.counters.findAndModify(
          {
            query: { _id: name },
            update: { $inc: { seq: 1 } },
            new: true
          }
   );

   return ret.seq;
}

// Testing
db.users.insert(
   {
     _id: getNextSequence("userid"),
     name: "Sarah C."
   }
);

db.users.insert(
   {
     _id: getNextSequence("userid"),
     name: "Bob D."
   }
);