MongoDB group with multiple id

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}
       }
     }
 ]);

Query:

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