Group query upon nested object in MongoDB?

To group documents by fields within nested objects in MongoDB, use dot notation with the $group aggregation stage. The dot notation allows you to access nested fields using the format "parentField.nestedField".

Syntax

db.collection.aggregate([
    { 
        $group: { 
            _id: "$parentField.nestedField", 
            count: { $sum: 1 } 
        } 
    }
]);

Create Sample Data

db.demo617.insertMany([
    {
        "clientDetails": {
            "Name": "Chris",
            "Age": 32,
            "Project": "Online Library Management System"
        }
    },
    {
        "clientDetails": {
            "Name": "David",
            "Age": 34,
            "Project": "Online Hospital Management System"
        }
    },
    {
        "clientDetails": {
            "Name": "David",
            "Age": 34,
            "Project": "Online Library Management System"
        }
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e99d2b86c954c74be91e69b"),
        ObjectId("5e99d2b86c954c74be91e69c"),
        ObjectId("5e99d2b96c954c74be91e69d")
    ]
}

View Sample Data

db.demo617.find();

{
    "_id": ObjectId("5e99d2b86c954c74be91e69b"),
    "clientDetails": {
        "Name": "Chris",
        "Age": 32,
        "Project": "Online Library Management System"
    }
}
{
    "_id": ObjectId("5e99d2b86c954c74be91e69c"),
    "clientDetails": {
        "Name": "David",
        "Age": 34,
        "Project": "Online Hospital Management System"
    }
}
{
    "_id": ObjectId("5e99d2b96c954c74be91e69d"),
    "clientDetails": {
        "Name": "David",
        "Age": 34,
        "Project": "Online Library Management System"
    }
}

Group by Nested Field

Group documents by the Project field within the clientDetails nested object ?

db.demo617.aggregate([
    { 
        $group: { 
            _id: "$clientDetails.Project", 
            count: { $sum: 1 } 
        } 
    }
]);

{ "_id": "Online Hospital Management System", "count": 1 }
{ "_id": "Online Library Management System", "count": 2 }

Conclusion

Use dot notation with $group to group documents by nested object fields. The format "$parentField.nestedField" allows MongoDB to access and group by any field within nested objects efficiently.