You are viewing an offline version of MongoDB documentation. Some page features might be unavailable. To view the latest version of the page or use interactive features, visit the live page.

$rand

Definition

$rand

Returns a pseudo-random floating point number in the interval [0, 1). 0 included, 1 is excluded.

Syntax

$rand has the following syntax:

{ $rand: {} }

Examples

Generate Random Data Points

The examples in this section illustrate charitable donations. The following collection contains a list of donors:

db.donors.insertMany(
   [
     { donorId: 1000, amount: 0, frequency: 1 },
     { donorId: 1001, amount: 0, frequency: 2 },
     { donorId: 1002, amount: 0, frequency: 1 },
     { donorId: 1003, amount: 0, frequency: 2 },
     { donorId: 1004, amount: 0, frequency: 1 }
   ]
)

The following example updates each document in the donors collection with a random donation amount:

db.donors.updateMany(
   {},
   [
      { $set:
         { amount:
            { $floor:
               { $multiply: [ { $rand: {} }, 100 ] }
            }
         }
      }
    ]
)

The empty update filter matches every document in the collection.

For each document we generate a value between 0 and 1 using $rand then scale the value with $multiply.

The $floor operator removes the decimal portion so the updated amount is an integer value.

After updating the collection, the documents look like this:

{ "donorId" : 1000, "amount" : 2, "frequency" : 1 }
{ "donorId" : 1001, "amount" : 58, "frequency" : 2 }
{ "donorId" : 1002, "amount" : 27, "frequency" : 1 }
{ "donorId" : 1003, "amount" : 26, "frequency" : 2 }
{ "donorId" : 1004, "amount" : 42, "frequency" : 1 }

Select Random Items From a Collection

The $rand operator can be used to select random documents from a collection. Given a collection of voter records:

db.voters.insertMany(
   [
     { name: "Archibald", voterId: 4321, district: 3, registered: true },
     { name: "Beckham", voterId: 4331, district: 3, registered: true },
     { name: "Carolin", voterId: 5321, district: 4, registered: true },
     { name: "Debarge", voterId: 4343, district: 3, registered: false },
     { name: "Eckhard", voterId: 4161, district: 3, registered: false },
     { name: "Faberge", voterId: 4300, district: 1, registered: true },
     { name: "Grimwald", voterId: 4111, district: 3, registered: true },
     { name: "Humphrey", voterId: 2021, district: 3, registered: true },
     { name: "Idelfon", voterId: 1021, district: 4, registered: true },
     { name: "Justo", voterId: 9891, district: 3, registered: false }
   ]
)

The following query retrieves a random selection of approximately half of the voters in district 3:

db.voters.find(
   {  district: 3,
      $expr: { $lt: [0.5, {$rand: {} } ] }
   },
   { _id: 0, name: 1, registered: 1 }
)

The intial match on the district field selects documents where the voter is from district 3.

The $expr operator uses $rand to further refine the find operation. For each document, $rand generates a value between 0 and 1. The threshold of 0.5 means the less than ($lt) comparison will be true for about half the documents in the set.

There are 7 voters in District 3, running the code selects about half of them.

{ "name" : "Beckham", "registered" : true }
{ "name" : "Eckhard", "registered" : false }
{ "name" : "Grimwald", "registered" : true }
{ "name" : "Humphrey", "registered" : true }

Learn More

$rand (aggregation)

Back

$natural

Update