$slice - Amazon DocumentDB
範例 (MongoDB Shell)程式碼範例

本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

$slice

$slice 彙總運算子可讓您從陣列的開頭或結尾周遊陣列，以傳回陣列的子集。這用於顯示陣列欄位中有限數量的項目，例如頂端或底部 N 個項目。

參數

  • array：要分割的陣列欄位。

  • n：整數，指定要傳回的元素數目。正值從陣列的開頭開始，負值則從陣列的結尾開始。

範例 (MongoDB Shell)

下列範例示範如何使用 $slice傳回每個 Chef 的前兩個最愛甜食。

建立範例文件

db.sweets.insertMany([
  { "_id" : 1, "name" : "Alvin", "favorites": [ "chocolate", "cake", "toffee", "beignets" ] },
  { "_id" : 2, "name" : "Tom", "favorites": [ "donuts", "pudding", "pie" ] },
  { "_id" : 3, "name" : "Jessica", "favorites": [ "fudge", "smores", "pudding", "cupcakes" ] },
  { "_id" : 4, "name" : "Rachel", "favorites": [ "ice cream" ] }
]);

查詢範例

db.sweets.aggregate([
  { $project: { _id: 0, name: 1, topTwoFavorites: { $slice: [ "$favorites", 2 ] } } }
]);

輸出

[
  { name: 'Alvin', topTwoFavorites: [ 'chocolate', 'cake' ] },
  { name: 'Tom', topTwoFavorites: [ 'donuts', 'pudding' ] },
  { name: 'Jessica', topTwoFavorites: [ 'fudge', 'smores' ] },
  { name: 'Rachel', topTwoFavorites: [ 'ice cream' ] }
]

在此範例中，運算$slice子用於從每個文件的favorites陣列中擷取前兩個元素。

程式碼範例

若要檢視使用 $slice命令的程式碼範例，請選擇您要使用的語言標籤：

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('sweets');

  const result = await collection.aggregate([
    { $project: { name: 1, topTwoFavorites: { $slice: ['$favorites', 2] } } }
  ]).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['sweets']

    result = list(collection.aggregate([
        { '$project': { 'name': 1, 'topTwoFavorites': { '$slice': ['$favorites', 2] } } }
    ]))

    print(result)
    client.close()

example()