MongoDB query to fetch random value using Map Reduce concept.

For random value with Map Reduce, use mapReduce() concept along with Math.random(). Let us create a collection with documents −

> 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") }

Display all documents from a collection with the help of find() method −

> db.demo651.find();

This will produce the following output −

{ "_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 }

Following is the query to fetch data −

> map = function() { ... ...    if (Math.random() < 0.1) { ...       emit(this._id, this); ...    } ... } function () {    if (Math.random() < 0.1) {       emit(this._id, this);    } } > > reduce = function(key, values) { ... return values; ... } 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();

This will produce the following output −

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