Push query results into variable with MongoDB?

To push query results into a variable in MongoDB, use the aggregate() method combined with toArray() to store the result in a JavaScript variable. This technique is useful for storing intermediate results for further processing.

Syntax

var queryResult = db.collection.aggregate([
    { $group: { "_id": null, "fieldName": { "$operation": "$field" } } }
]);
var variableName = queryResult.toArray()[0]["fieldName"];

Sample Data

db.demo142.insertMany([
    {"Value": 50},
    {"Value": 45},
    {"Value": 60},
    {"Value": 55},
    {"Value": 50}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e32e9c6fdf09dd6d08539b2"),
        ObjectId("5e32e9cafdf09dd6d08539b3"),
        ObjectId("5e32e9cdfdf09dd6d08539b4"),
        ObjectId("5e32e9d0fdf09dd6d08539b5"),
        ObjectId("5e32e9d9fdf09dd6d08539b6")
    ]
}

Display Documents

db.demo142.find();

{ "_id": ObjectId("5e32e9c6fdf09dd6d08539b2"), "Value": 50 }
{ "_id": ObjectId("5e32e9cafdf09dd6d08539b3"), "Value": 45 }
{ "_id": ObjectId("5e32e9cdfdf09dd6d08539b4"), "Value": 60 }
{ "_id": ObjectId("5e32e9d0fdf09dd6d08539b5"), "Value": 55 }
{ "_id": ObjectId("5e32e9d9fdf09dd6d08539b6"), "Value": 50 }

Example: Store Maximum Value in Variable

Find the maximum value and store it in a JavaScript variable ?

var query = db.demo142.aggregate([
    { "$group": { "_id": null, "MaxValue": { "$max": "$Value" } } }
]);
var largestValue = query.toArray()[0]["MaxValue"];
printjson(largestValue);

60

How It Works

• aggregate() returns a cursor object containing the aggregation results
• toArray() converts the cursor to an array of documents
• [0] accesses the first (and only) document in the result
• ["MaxValue"] extracts the specific field value

Conclusion

Use aggregate().toArray()[0]["fieldName"] to store query results in variables. This approach works with any aggregation operation and allows you to use the results in subsequent MongoDB operations or JavaScript logic.