MongoDB: นำผลลัพธ์ Aggregate ไปเก็บในตัวแปร

สร้างคอลเลกชันตัวอย่าง

> db.demo142.insertOne({"Value":50});
{ "acknowledged": true, "insertedId": ObjectId("5e32e9c6fdf09dd6d08539b2") }
> db.demo142.insertOne({"Value":45});
{ "acknowledged": true, "insertedId": ObjectId("5e32e9cafdf09dd6d08539b3") }
> db.demo142.insertOne({"Value":60});
{ "acknowledged": true, "insertedId": ObjectId("5e32e9cdfdf09dd6d08539b4") }
> db.demo142.insertOne({"Value":55});
{ "acknowledged": true, "insertedId": ObjectId("5e32e9d0fdf09dd6d08539b5") }
> db.demo142.insertOne({"Value":50});
{ "acknowledged": true, "insertedId": ObjectId("5e32e9d9fdf09dd6d08539b6") }

แสดงข้อมูล

> 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 }

คิวรี: Aggregate หา Max แล้วเก็บในตัวแปร

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

ผลลัพธ์: 60

60