How to add or inc to an array in a update query? - mongodb

Question

If I have a doc which has an array which contains a items which represents counts for a day, perhaps like :-
{
data : [ {'20141102' : 2 },{'20141103' : 4 } ]
}
when I do an update, and I have a string '20141103' and then later a '20141104' I want to either inc the array entry or add a new array entry. Is this possible with an update?

Answer 1

Yes, it's feasible. I tried like this:
(run on mongo shell; both client and server are V2.6.4)
function tryAndFine(coll, key, value) {
var entry = {};
entry[key] = value;
var parent = 'data';
var prefix = parent + '.';
function incrementOnly() {
var criteria = {};
criteria[prefix + key] = {$exists : true};
var update = {};
update[prefix + "$." + key] = value;
var result = coll.update(criteria, {$inc : update});
// if increment fails, try to add a new one
if (result.nModified == 0) {
addNewElement();
}
}
function addNewElement() {
var criteria = {};
criteria[prefix + key] = {$exists : false};
var update = {};
update[parent] = entry;
var result = coll.update(criteria, {$push : update}, {upsert : true});
// if exists, try to increment the count
if (result.upserted == 0 && result.nModified == 0) {
incrementOnly();
}
}
// run entry
incrementOnly();
}
// test
var c = db.c;
c.drop();
tryAndFine(c, '20141103', 1);
tryAndFine(c, '20141103', 1);
tryAndFine(c, '20141104', 1);
tryAndFine(c, '20141105', 1);
tryAndFine(c, '20141104', 1);
// output
{
"_id" : ObjectId("54577e1a3502852bd4ad2395"),
"data" : [ {
"20141103" : 2
}, {
"20141104" : 2
}, {
"20141105" : 1
} ]
}