$position

Amazon DocumentDB 中的$position修饰符指定$push运算符在数组中插入元素的位置。如果不使用$position修饰$push符，则运算符将元素插入数组的末尾。

参数

• field：要更新的数组字段。

• num: 数组中应插入元素的位置，基于从零开始的索引。

注意：要使用$position修饰符，它必须与修$each饰符一起出现。

示例（MongoDB 外壳）

以下示例演示如何使用$position操作员在项目管理系统的特定位置插入任务。

创建示例文档

db.projects.insertOne({ "_id": 1, "name": "Website Redesign", "tasks": ["Design mockups"] })

查询示例 1-在开头添加紧急任务

db.projects.updateOne(
   { _id: 1 },
   {
     $push: {
        tasks: {
           $each: ["Security audit", "Performance review"],
           $position: 0
        }
     }
   }
)

输出 1

{ "_id": 1, "name": "Website Redesign", "tasks": ["Security audit", "Performance review", "Design mockups"] }

查询示例 2-在特定位置添加任务

db.projects.insertOne({ "_id": 2, "name": "Mobile App", "tasks": ["Setup project", "Create wireframes", "Deploy to store"] })

db.projects.updateOne(
   { _id: 2 },
   {
     $push: {
        tasks: {
           $each: ["Code review", "Testing phase"],
           $position: 2
        }
     }
   }
)

输出 2

{ "_id": 2, "name": "Mobile App", "tasks": ["Setup project", "Create wireframes", "Code review", "Testing phase", "Deploy to store"] }

代码示例

要查看使用该$position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 insertTasksAtPosition() {
  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('projects');

  await collection.updateOne(
    { _id: 1 },
    {
      $push: {
        tasks: {
          $each: ["Security audit", "Performance review"],
          $position: 0
        }
      }
    }
  );

  const updatedProject = await collection.findOne({ _id: 1 });
  console.log(updatedProject);

  await client.close();
}

insertTasksAtPosition();

Python

from pymongo import MongoClient

def insert_tasks_at_position():
    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['projects']

    result = collection.update_one(
        {'_id': 1},
        {
            '$push': {
                'tasks': {
                    '$each': ['Security audit', 'Performance review'],
                    '$position': 0
                }
            }
        }
    )

    updated_project = collection.find_one({'_id': 1})
    print(updated_project)

    client.close()

insert_tasks_at_position()