MongoDb - Add value to the existing integer field in a query

Question

What i am doing is -

db.getCollection('campaigns').update(
{'_id' : ObjectId("57aea88f2f1321710f7373f8")},
{ $set : {
        _a :  'this._a' + 1000
    }
}
);

I tried this also :

db.getCollection('campaigns').update(
{'_id' : ObjectId("57aea88f2f1321710f7373f8")},
{ $set : {
        _a :  this._a + 1000
    }
}
);

What i am getting is _a = nan.

Please help me in this. I am very new to MongoDb. What I need is add value in _a field with 1000.

Answer 1

db.campaigns.update(
   { '_id' : ObjectId("57aea88f2f1321710f7373f8") },
   { $inc: { _a: 1000 } });