MongoDB group with multiple id

Question

I have a set of documents with more than 20 keys in each of them, and the keys vary with documents. Some keys may not be present in all the documents. I am trying to run a MongoDB group operation using the aggregation framework. The query looks something like this -

db.collection.aggregate([{'$group': {'count': {'$sum': 1}, '_id': {'location': '$location', 'type': '$type', 'language': '$language'}}}])

In ideal case, it should return the documents where 3 of the keys should be present and perform a "group-by" operation on them. But the result looks something like this -

{
    "result" : [
        {
            "_id" : {
                "location" : "abc",
                "type" : "456"
            },
            "count" : 5
        },
        {
            "_id" : {
                "type" : "123",
                "language" : "english"
            },
            "count" : 1
        },
        {
            "_id" : {
                "location" : "ghi",
                "type" : "9876",
                "language" : "latin"
            },
            "count" : 2
        },
        {
            "_id" : {
                "language" : "hebrew",
                "type" : "9434"
            },
            "count" : 3
        },
        {
            "_id" : {
                "type" : "525",
                "location" : "cari"
            },
            "count" : 1
        },
        {
            "_id" : {
                "language" : "spanish",
                "location" : "dff"
            },
            "count" : 12
        },
        {
            "_id" : {
                "location" : "adpj",
                "type" : "3463",
                            "language": "english"
            },
            "count" : 8
        },
        {
            "_id" : {
                "language" : "french",
                "location" : "nts"
            },
            "count" : 6
        }
    ],
    "ok" : 1
}

The problem is, MongoDB does the group operation even when it does not find all 3 keys I asked for in the query and shows partial group by. I am only interested in the results where I get all the keys. Filtering in the client side is not an option. Can anyone help out?

Answer 1

For MongoDB's $group-operator, no value is also a value.

When you want to exclude any documents where not all three keys are present, you can add a $match-step to your aggregation pipeline which filters any documents which do not have all these keys.

 db.collection.aggregate([
     { $match: { 
         "type" : { "$exists" : true}, 
         "location" : { "$exists" : true}, 
         "language" : { "$exists" : true}
       } 
     },
     { $group: {
         "_id": {
             "location": "$location", 
             "type": "$typ", 
             "language": "$language"
         },
         "count": {$sum: 1}
       }
     }
 ]);

Playground link: https://mongoplayground.net/p/WeM_NQlgAHs

Answer 2

Query:

db.collection.aggregate([{
    $match: {
        type: {
            "$exists": true
        },
        location: {
            "$exists": true
        },
        language: {
            "$exists": true
        }
    }
}])