$rand

8.0 版的新增内容

Amazon DocumentDB 中的$rand运算符用于生成 0 到 1 之间的随机数。

参数

无

示例（MongoDB 外壳）

以下示例演示如何使用$rand运算符从temp集合中随机选择两个文档。

创建示例文档

db.items.insertMany([
  { "name": "pencil", "quantity": 110 },
  { "name": "pen", "quantity": 159 }
])

查询示例

db.items.aggregate([
  {
    $project: {
      randomValue: { $rand: {} }
    }
  }
])

输出

[
  {
    _id: ObjectId('6924a5edd66dcae121d29517'),
    randomValue: 0.8615243955294392
  },
  {
    _id: ObjectId('6924a5edd66dcae121d29518'),
    randomValue: 0.22815483022099903
  }
]

代码示例

要查看使用该$rand命令的代码示例，请选择要使用的语言的选项卡：

Node.js

const { MongoClient } = require('mongodb');

async function example() {
  const client = await MongoClient.connect('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');
  const db = client.db('test');
  const collection = db.collection('items');

  const result = await collection.aggregate([
    {
      $project: {
        randomValue: { $rand: {} }
      }
    }
  ]).toArray();

  console.log(result);

  await client.close();
}

example();

Python

from pymongo import MongoClient

def example():
  client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
  db = client['test']
  collection = db['items']

  result = list(collection.aggregate([
      {
        "$project": {
          "randomValue": { "$rand": {} }
        }
      }
  ]))

  print(result)

  client.close()

example()