Mongodb Aggregation Framework | Group over multiple values?

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OK, so the solution is to specify an aggregate key for the _id value. This is documented here as:

You can specify a single field from the documents in the pipeline, a previously computed value, or an aggregate key made up from several incoming fields.

But it doesn’t actually define the format for an aggregate key. Reading the earlier documentation here I saw that the previous collection.group method could take multiple fields and that the same structure is used in the new framework.

So, to group over multiple fields you could use _id : { success:'$success', responseCode:'$responseCode', label:'$label'}

As in:

resultsCollection.aggregate(
{ $match : { testid : testid} },
{ $skip : alreadyRead },
{ $project : {
        timeStamp : 1 ,
        label : 1,
        responseCode : 1 ,
        value : 1,
        success : 1
    }},
{ $group : {
        _id :  { success:'$success', responseCode:'$responseCode', label:'$label'},
        max_timeStamp : { $timeStamp : 1 },
        count_responseCode : { $sum : 1 },
        avg_value : { $sum : "$value" },
        count_success : { $sum : 1 }
    }}
);

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