Query subset of fields based on count criteria in mongodb

Question

My stored documents in a collection are in the format:


    {
        "a": {
            ...

            "b": {
                "c": cc,
                "d": dd,
                "e": ee,
                "f": ff
            }
        }

    }

I want to get fields "d" and "e" from all documents in the collection where count(c) > 1.

How can this be done efficiently?

So, I'm counting how many time does field "c" has value "cc" over all the documents in collection. If that count is > 1, I want to get all of those documents. — Sitesh Shrivastava, Nov 16 '12 at 09:53
So you want all documents which have a.b.c = "cc", unless it's only a single document? Wouldn't it be an option to handle this exception on the application level? — Philipp, Nov 16 '12 at 09:58
Yes. But, Haven't found any way to do this thing till now :( — Sitesh Shrivastava, Nov 16 '12 at 10:07

score 0 · Accepted Answer · answered Nov 16 '12 at 16:34

Arrived at a map-reduce solution for the problem:

My functions look like this now:

m = function() {
    emit(this.a.b.c, { data: [[this.a.b.d, this.a.b.e]] });
}

r = function(key, values) {
    var result = [];
    values.forEach(function(V) {
        Array.prototype.push.apply(result, V.data);
    });
    return {data: result};
}

db.coll.mapReduce(m, r, {out: {replace: "data"}})

Works out great for this purpose :)

Query subset of fields based on count criteria in mongodb

1 Answers1