$rand

Generate Random Data Points生成随机数据点

This example models charitable donations. 这个例子是慈善捐赠的典范。The collection starts with a list of donors.集合从捐赠者名单开始。

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 }
   ]
)

Then we construct an operation to update each document with a random donation amount:然后我们构造一个操作，用随机捐赠金额更新每个文档：

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

The empty update filter matches every document in the collection.空更新筛选器匹配集合中的每个文档。

For each document we generate a value between 0 and 1 using $rand then scale the value with $multiply.对于每个文档，我们使用$rand生成一个介于0和1之间的值，然后使用$multiply缩放该值。

The $floor operator removes the decimal portion so the updated amount is an integer value.$floor运算符删除小数部分，因此更新的金额是一个整数值。

After updating the collection, the documents look like this:更新集合后，文档如下所示：

{ "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从集合中随机选择项目

The $rand operator can be used to select random documents from a collection. $rand运算符可用于从集合中随机选择文档。Given a collection of voter records:给定一组选民记录：

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 }
   ]
)

Imagine you want to select about half of the voters in District 3 to do some polling.想象一下，你想选择第三区大约一半的选民进行投票。

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.district字段上的初始匹配选择选民来自选区3的文件。

The $expr operator uses $rand to further refine the find operation. $expr运算符使用$rand进一步优化查找操作。For each document, $rand generates a value between 0 and 1. 对于每个文档，$rand生成一个介于0和1之间的值。The threshold of 0.5 means the less than ($lt) comparison will be true for about half the documents in the set.阈值为0.5意味着集合中大约一半的文档都可以进行小于（($lt)）的比较。

There are 7 voters in District 3, running the code selects about half of them.第三区有7名选民，运行该代码可以选择其中大约一半的选民。

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