How to Make an Auto Incrementing Field

Generally in MongoDB, one does not use an auto-increment pattern for _id's (or other fields), as this does not scale up well on large database clusters. Instead one typically uses Object IDs.

Side counter method

One can keep a counter of the current _id in a side document, in a collection dedicated to counters.
Then use FindAndModify to atomically obtain an id and increment the counter.

> db.counters.insert({_id: "userId", c: 0});

> var o = db.counters.findAndModify(
...        {query: {_id: "userId"}, update: {$inc: {c: 1}}});
{ "_id" : "userId", "c" : 0 }
> db.mycollection.insert({_id:o.c, stuff:"abc"});

> o = db.counters.findAndModify(
...        {query: {_id: "userId"}, update: {$inc: {c: 1}}});
{ "_id" : "userId", "c" : 1 }
> db.mycollection.insert({_id:o.c, stuff:"another one"});

Once you obtain the next id in the client, you can use it and be sure no other client has it.

Optimistic loop method

One can do it with an optimistic concurrency "insert if not present" loop. The following example, in Mongo shell Javascript syntax, demonstrates.

// insert incrementing _id values into a collection
function insertObject(o) {
    x = db.myCollection;
    while( 1 ) {
        // determine next _id value to try
        var c = x.find({},{_id:1}).sort({_id:-1}).limit(1);
        var i = c.hasNext() ? c.next()._id + 1 : 1;
        o._id = i;
        x.insert(o);
        var err = db.getLastErrorObj();
        if( err && err.code ) {
            if( err.code == 11000 /* dup key */ )
                continue;
            else
                print("unexpected error inserting data: " + tojson(err));
        }
        break;
    }
}

The above should work well unless there is an extremely high concurrent insert rate on the collection. In that case, there would be a lot of looping potentially.

How to Make an Auto Incrementing Field

Side counter method

Optimistic loop method

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