MongoDB - Aggregation, group by an array value

Question

I have the following document structure:

{
    _id: ...,
    name: "Item1",
    Props: [
    {
        Key: "numberKey",
        Val: 1234
    },

    {
        Key: "dateKey",
        Val: Date("2013-09-09")
    }]
}

This is simplified and there can be various Keys and Values in Props field in the real application.

My question - is it possible to $group and $sum "numberKey"s by "dateKey"s?

What structure should I use if this is not possible? I need users to let add keys and values so I need something flexible.

Charlie Page · Accepted Answer · 2013-09-13 18:40:31Z

2

Unfortunately, that isn't possible using aggregation with your schema. The problem is that aggregation is meant to operate over values in an array that are being selected by the $group clause and those elements have all the data needed. Your setup separates what you want to group by and what you want to sum. You could use a mapReduce job to do what you want with your schema. http://docs.mongodb.org/manual/core/map-reduce/ should be able to get you started.

Let me know if you have any other questions.

Best, Charlie

answered Sep 13, 2013 at 18:40

Charlie Page

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1 Comment

nxy Over a year ago

Thanks. I need this in the real-time so I'm not sure if mapReduce is the way. I decided to keep this schema and duplicate keys-values as fields for the aggregation purpose. I don't like this but it is probably the best way if I want to keep low count of indexes and be able to aggregate values at the same time.

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