MongoDB aggregation with multiple keys

MongoDB aggregation with multiple keys allows you to group documents by several fields simultaneously. Use the $group stage with a composite _id object containing multiple grouping fields.

Syntax

db.collection.aggregate([
    {
        $group: {
            _id: {
                field1: "$field1",
                field2: "$field2"
            },
            aggregatedField: { $operator: "$fieldToAggregate" }
        }
    }
]);

Sample Data

db.demo190.insertMany([
    {
        "DueDate": ISODate("2020-01-01"),
        "Value": 10,
        "Name": "Chris"
    },
    {
        "DueDate": ISODate("2020-02-05"),
        "Value": 30,
        "Name": "David"
    },
    {
        "DueDate": ISODate("2020-01-01"),
        "Value": 40,
        "Name": "Chris"
    }
]);

Display all documents from the collection ?

db.demo190.find();

{ "_id": ObjectId("5e3ad76403d395bdc21346bf"), "DueDate": ISODate("2020-01-01T00:00:00Z"), "Value": 10, "Name": "Chris" }
{ "_id": ObjectId("5e3ad76403d395bdc21346c0"), "DueDate": ISODate("2020-02-05T00:00:00Z"), "Value": 30, "Name": "David" }
{ "_id": ObjectId("5e3ad7f003d395bdc21346c1"), "DueDate": ISODate("2020-01-01T00:00:00Z"), "Value": 40, "Name": "Chris" }

Example: Group by Name and Year

Group documents by Name and extract the year from DueDate, then sum the Values ?

db.demo190.aggregate([
    {
        $group: {
            _id: {
                Name: "$Name",
                DueDate: { $year: "$DueDate" }
            },
            Value: { $sum: "$Value" }
        }
    }
]);

{ "_id": { "Name": "David", "DueDate": 2020 }, "Value": 30 }
{ "_id": { "Name": "Chris", "DueDate": 2020 }, "Value": 50 }

How It Works

• The _id field contains an object with multiple grouping keys
• $year extracts the year portion from the DueDate field
• $sum aggregates the Value field for each group combination
• Documents with the same Name and year are grouped together

Conclusion

Use composite _id objects in $group to aggregate by multiple fields. This enables complex grouping scenarios where documents must match on several criteria simultaneously.