Why does my MongoDB group query return always 0 in float conversion? How to fix it?

When MongoDB group queries return 0 during float conversion, it's typically because string values aren't being properly converted to numbers. Use parseFloat() or MongoDB's $toDouble operator to convert string numbers to actual numeric types.

Syntax

// Using parseFloat() in forEach
db.collection.find({}).forEach(function(doc) {
    doc.field = parseFloat(doc.field);
    db.collection.save(doc);
});

// Using $toDouble in aggregation
db.collection.aggregate([
    { $addFields: { "field": { $toDouble: "$field" } } }
]);

Sample Data

db.demo523.insertOne({
    "details": {
        "values": "-0.45"
    }
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e89b7efb3fbf26334ef611f")
}

db.demo523.find();

{ "_id": ObjectId("5e89b7efb3fbf26334ef611f"), "details": { "values": "-0.45" } }

Method 1: Using parseFloat() with forEach

Convert string values to float numbers using parseFloat() ?

db.demo523.find({}).forEach(function(d) {
    d.details.values = parseFloat(d.details.values);
    db.demo523.save(d);
});

Verify the conversion ?

db.demo523.find();

{ "_id": ObjectId("5e89b7efb3fbf26334ef611f"), "details": { "values": -0.45 } }

Method 2: Using $toDouble in Aggregation

Use MongoDB's built-in $toDouble operator for safer conversion ?

db.demo523.aggregate([
    {
        $addFields: {
            "details.values": { $toDouble: "$details.values" }
        }
    },
    { $out: "demo523" }
]);

Key Points

• String values like "-0.45" must be converted to numeric types before mathematical operations
• parseFloat() works in JavaScript operations but $toDouble is preferred in aggregation pipelines
• Always verify data types after conversion to ensure proper numeric operations

Conclusion

MongoDB group queries return 0 in float conversion when string values aren't properly converted to numbers. Use parseFloat() with forEach() or $toDouble in aggregation pipelines for reliable numeric conversions.