MongoDB Query ดึงค่า Random ด้วย Map Reduce

การดึงค่า Random ด้วย Map Reduce

สำหรับค่า Random ด้วย Map Reduce ให้ใช้ mapReduce() ร่วมกับ Math.random()

สร้าง Collection พร้อมข้อมูล

> db.demo651.insertOne({Value:10});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f0330e3c3cd0dcff36a57") }
> db.demo651.insertOne({Value:20});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f0332e3c3cd0dcff36a58") }
> db.demo651.insertOne({Value:30});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f0335e3c3cd0dcff36a59") }
> db.demo651.insertOne({Value:40});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f0337e3c3cd0dcff36a5a") }
> db.demo651.insertOne({Value:50});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f0339e3c3cd0dcff36a5b") }
> db.demo651.insertOne({Value:60});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f033be3c3cd0dcff36a5c") }
> db.demo651.insertOne({Value:70});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f033ee3c3cd0dcff36a5d") }
> db.demo651.insertOne({Value:80});
{ "acknowledged" : true, "insertedId" : ObjectId("5e9f0340e3c3cd0dcff36a5e") }

แสดงเอกสารทั้งหมด

> db.demo651.find();

ผลลัพธ์:

{ "_id" : ObjectId("5e9f0330e3c3cd0dcff36a57"), "Value" : 10 }
{ "_id" : ObjectId("5e9f0332e3c3cd0dcff36a58"), "Value" : 20 }
{ "_id" : ObjectId("5e9f0335e3c3cd0dcff36a59"), "Value" : 30 }
{ "_id" : ObjectId("5e9f0337e3c3cd0dcff36a5a"), "Value" : 40 }
{ "_id" : ObjectId("5e9f0339e3c3cd0dcff36a5b"), "Value" : 50 }
{ "_id" : ObjectId("5e9f033be3c3cd0dcff36a5c"), "Value" : 60 }
{ "_id" : ObjectId("5e9f033ee3c3cd0dcff36a5d"), "Value" : 70 }
{ "_id" : ObjectId("5e9f0340e3c3cd0dcff36a5e"), "Value" : 80 }

คิวรี Map Reduce สุ่มข้อมูล (โอกาส 10%)

> map = function() {
...     if (Math.random() < 0.1) {
...         emit(this._id, this);
...     }
... }
> reduce = function(key, values) {
...     return values;
... }
> db.demo651.mapReduce( map, reduce, { out: 'demo_651' } );
{ "result" : "demo_651", "timeMillis" : 1104, "counts" : { "input" : 8, "emit" : 1, "reduce" : 0, "output" : 1 }, "ok" : 1 }
> db.demo_651.find();

ผลลัพธ์ (ตัวอย่าง - อาจแตกต่างไปทุกครั้ง):

{ "_id" : ObjectId("5e9f033be3c3cd0dcff36a5c"), "value" : { "_id" : ObjectId("5e9f033be3c3cd0dcff36a5c"), "Value" : 60 } }