MongoDB Aggregate sum of values in a list of dictionaries for all documents?

To aggregate sum of values in a list of dictionaries for all documents, use $unwind to flatten the array, then $group with $sum operators to calculate totals by grouping field.

Syntax

db.collection.aggregate([
    { $unwind: "$arrayField" },
    { 
        $group: {
            _id: "$arrayField.groupByField",
            "fieldSum1": { $sum: "$arrayField.field1" },
            "fieldSum2": { $sum: "$arrayField.field2" }
        }
    }
]);

Sample Data

db.demo117.insertMany([
    {
        ID: 101,
        Details: [
            {
                Name: "Chris",
                Value1: 20,
                Value2: 30,
                Value3: 10
            },
            {
                Name: "David",
                Value1: 10,
                Value2: 50,
                Value3: 30
            }
        ]
    },
    {
        ID: 102,
        Details: [
            {
                Name: "Chris",
                Value1: 20,
                Value2: 10,
                Value3: 20
            },
            {
                Name: "David",
                Value1: 30,
                Value2: 40,
                Value3: 10
            }
        ]
    }
]);

Example

Aggregate sum of values by Name across all documents ?

db.demo117.aggregate([
    {
        $unwind: "$Details"
    },
    {
        $group: {
            _id: "$Details.Name",
            "Value1": { $sum: "$Details.Value1" },
            "Value2": { $sum: "$Details.Value2" },
            "Value3": { $sum: "$Details.Value3" }
        }
    }
]);

{ "_id": "David", "Value1": 40, "Value2": 90, "Value3": 40 }
{ "_id": "Chris", "Value1": 40, "Value2": 40, "Value3": 30 }

How It Works

• $unwind creates a separate document for each element in the Details array
• $group groups documents by Name and sums the corresponding values
• $sum calculates the total for each numeric field across all matching documents

Conclusion

Use $unwind followed by $group with $sum to aggregate values from nested arrays. This approach effectively flattens array data and calculates totals by grouping criteria.