How can I aggregate collection and group by field count in MongoDB?

In MongoDB, use the $group stage with aggregate() to group documents by field values and count occurrences. The $sum operator with value 1 counts documents in each group.

Syntax

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

Sample Data

db.demo616.insertMany([
    { "details": { "Name": "Chris", "Age": 21 } },
    { "details": { "Name": "Chris", "Age": 22 } },
    { "details": { "Name": "Bob", "Age": 23 } },
    { "details": { "Name": "Sam", "Age": 21 } },
    { "details": { "Name": "Chris", "Age": 24 } }
]);

Display Collection Data

db.demo616.find();

{ "_id" : ObjectId("5e99bfac65492f6c60d00283"), "details" : { "Name" : "Chris", "Age" : 21 } }
{ "_id" : ObjectId("5e99bfb065492f6c60d00284"), "details" : { "Name" : "Chris", "Age" : 22 } }
{ "_id" : ObjectId("5e99bfb865492f6c60d00285"), "details" : { "Name" : "Bob", "Age" : 23 } }
{ "_id" : ObjectId("5e99bfbd65492f6c60d00286"), "details" : { "Name" : "Sam", "Age" : 21 } }
{ "_id" : ObjectId("5e99bfc165492f6c60d00287"), "details" : { "Name" : "Chris", "Age" : 24 } }

Group by Name and Count Documents

Following query groups documents by details.Name and counts occurrences ?

db.demo616.aggregate([
    {
        $group: {
            _id: "$details.Name",
            Total: { $sum: 1 }
        }
    }
]);

{ "_id" : "Sam", "Total" : 1 }
{ "_id" : "Bob", "Total" : 1 }
{ "_id" : "Chris", "Total" : 3 }

Key Points

• $group stage groups documents by the _id field value.
• $sum: 1 adds 1 for each document in the group, effectively counting them.
• Use dot notation like "$details.Name" to group by nested fields.

Conclusion

MongoDB's $group stage with $sum: 1 efficiently counts documents by field values. This aggregation pattern is essential for generating frequency statistics from your data.