$last

$lastOperator di Amazon DocumentDB digunakan untuk mengembalikan elemen terakhir dalam array yang cocok dengan kriteria kueri. Hal ini sangat berguna untuk mengambil elemen terbaru atau terakhir dalam array yang memenuhi kondisi tertentu.

Parameter

• expression: Ekspresi untuk mencocokkan elemen array.

Contoh (MongoDB Shell)

Contoh berikut menunjukkan penggunaan $last operator dalam kombinasi dengan $filter untuk mengambil elemen terakhir dari array yang memenuhi kondisi tertentu (misalnya, subjek adalah 'sains').

Buat dokumen sampel

db.collection.insertMany([
  {
    "_id": 1,
    "name": "John",
    "scores": [
      { "subject": "math", "score": 82 },
      { "subject": "english", "score": 85 },
      { "subject": "science", "score": 90 }
    ]
  },
  {
    "_id": 2,
    "name": "Jane",
    "scores": [
      { "subject": "math", "score": 92 },
      { "subject": "english", "score": 88 },
      { "subject": "science", "score": 87 }
    ]
  },
  {
    "_id": 3,
    "name": "Bob",
    "scores": [
      { "subject": "math", "score": 75 },
      { "subject": "english", "score": 80 },
      { "subject": "science", "score": 85 }
    ]
  }
]);

Contoh kueri

db.collection.aggregate([
    { $match: { name: "John" } },
    {
      $project: {
        name: 1,
        lastScienceScore: {
          $last: {
            $filter: {
              input: "$scores",
              as: "score",
              cond: { $eq: ["$$score.subject", "science"] }
            }
          }
        }
      }
    }
  ]);

Keluaran

[
  {
    _id: 1,
    name: 'John',
    lastScienceScore: { subject: 'science', score: 90 }
  }
]

Contoh penggunaan operator jendela (MongoDB Shell)

Baru dari versi 8.0.2.

$lastOperator juga dapat digunakan sebagai operator jendela di pang $setWindowFields gung. Dalam konteks ini, ia mengembalikan nilai expression dari dokumen terakhir di setiap jendela. Anda menentukan operator di bawah output bidang, dan secara opsional menentukan batas jendela dengan window dokumen.

Buat dokumen sampel

db.stockPrices.insertMany([
  { _id: 1, ticker: "ABC", hour: 1, price: 50 },
  { _id: 2, ticker: "ABC", hour: 2, price: 45 },
  { _id: 3, ticker: "ABC", hour: 3, price: 60 },
  { _id: 4, ticker: "ABC", hour: 4, price: 40 },
  { _id: 5, ticker: "XYZ", hour: 1, price: 30 },
  { _id: 6, ticker: "XYZ", hour: 2, price: 35 },
  { _id: 7, ticker: "XYZ", hour: 3, price: 25 }
]);

Contoh kueri

Contoh berikut mempartisi dokumen berdasarkanticker, mengurutkan setiap partisi berdasarkanhour, dan mengembalikan harga terakhir (terbaru) yang dicatat di setiap partisi.

db.stockPrices.aggregate([
  {
    $setWindowFields: {
      partitionBy: "$ticker",
      sortBy: { hour: 1 },
      output: {
        lastPrice: {
          $last: "$price",
          window: { documents: ["unbounded", "unbounded"] }
        }
      }
    }
  }
]);

Keluaran

[
  { "_id": 1, "ticker": "ABC", "hour": 1, "price": 50, "lastPrice": 40 },
  { "_id": 2, "ticker": "ABC", "hour": 2, "price": 45, "lastPrice": 40 },
  { "_id": 3, "ticker": "ABC", "hour": 3, "price": 60, "lastPrice": 40 },
  { "_id": 4, "ticker": "ABC", "hour": 4, "price": 40, "lastPrice": 40 },
  { "_id": 5, "ticker": "XYZ", "hour": 1, "price": 30, "lastPrice": 25 },
  { "_id": 6, "ticker": "XYZ", "hour": 2, "price": 35, "lastPrice": 25 },
  { "_id": 7, "ticker": "XYZ", "hour": 3, "price": 25, "lastPrice": 25 }
]

Setiap dokumen ditambah denganlastPrice, harga dari dokumen terakhir di jendelanya (yang terbaru hour di partisi).

Contoh kode

Untuk melihat contoh kode untuk menggunakan $last operator, pilih tab untuk bahasa yang ingin Anda gunakan. Contoh berikut menunjukkan penggunaan ekspresi (in$project) dan penggunaan operator jendela (in$setWindowFields):

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');

  // Expression usage: last element of a filtered array
  const collection = db.collection('collection');
  await collection.insertMany([
    {
      _id: 1,
      name: "John",
      scores: [
        { subject: "math", score: 82 },
        { subject: "english", score: 85 },
        { subject: "science", score: 90 }
      ]
    },
    {
      _id: 2,
      name: "Jane",
      scores: [
        { subject: "math", score: 92 },
        { subject: "english", score: 88 },
        { subject: "science", score: 87 }
      ]
    },
    {
      _id: 3,
      name: "Bob",
      scores: [
        { subject: "math", score: 75 },
        { subject: "english", score: 80 },
        { subject: "science", score: 85 }
      ]
    }
  ]);
  const expressionResult = await collection.aggregate([
    { $match: { name: "John" } },
    {
      $project: {
        name: 1,
        lastScienceScore: {
          $last: {
            $filter: {
              input: "$scores",
              as: "score",
              cond: { $eq: ["$$score.subject", "science"] }
            }
          }
        }
      }
    }
  ]).toArray();
  console.log('Expression result:', JSON.stringify(expressionResult, null, 2));

  // Window operator usage: last value within each partition
  const stockPrices = db.collection('stockPrices');
  await stockPrices.insertMany([
    { _id: 1, ticker: "ABC", hour: 1, price: 50 },
    { _id: 2, ticker: "ABC", hour: 2, price: 45 },
    { _id: 3, ticker: "ABC", hour: 3, price: 60 },
    { _id: 4, ticker: "ABC", hour: 4, price: 40 },
    { _id: 5, ticker: "XYZ", hour: 1, price: 30 },
    { _id: 6, ticker: "XYZ", hour: 2, price: 35 },
    { _id: 7, ticker: "XYZ", hour: 3, price: 25 }
  ]);
  const windowResult = await stockPrices.aggregate([
    {
      $setWindowFields: {
        partitionBy: "$ticker",
        sortBy: { hour: 1 },
        output: {
          lastPrice: {
            $last: "$price",
            window: { documents: ["unbounded", "unbounded"] }
          }
        }
      }
    }
  ]).toArray();
  console.log('Window result:', JSON.stringify(windowResult, null, 2));

  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

    # Expression usage: last element of a filtered array
    collection = db.collection
    collection.insert_many([
        {
            "_id": 1,
            "name": "John",
            "scores": [
                { "subject": "math", "score": 82 },
                { "subject": "english", "score": 85 },
                { "subject": "science", "score": 90 }
            ]
        },
        {
            "_id": 2,
            "name": "Jane",
            "scores": [
                { "subject": "math", "score": 92 },
                { "subject": "english", "score": 88 },
                { "subject": "science", "score": 87 }
            ]
        },
        {
            "_id": 3,
            "name": "Bob",
            "scores": [
                { "subject": "math", "score": 75 },
                { "subject": "english", "score": 80 },
                { "subject": "science", "score": 85 }
            ]
        }
    ])
    expression_pipeline = [
        { "$match": { "name": "John" } },
        {
            "$project": {
                "name": 1,
                "lastScienceScore": {
                    "$last": {
                        "$filter": {
                            "input": "$scores",
                            "as": "score",
                            "cond": { "$eq": ["$$score.subject", "science"] }
                        }
                    }
                }
            }
        }
    ]
    expression_result = list(collection.aggregate(expression_pipeline))
    print('Expression result:', expression_result)

    # Window operator usage: last value within each partition
    stock_prices = db.stockPrices
    stock_prices.insert_many([
        { "_id": 1, "ticker": "ABC", "hour": 1, "price": 50 },
        { "_id": 2, "ticker": "ABC", "hour": 2, "price": 45 },
        { "_id": 3, "ticker": "ABC", "hour": 3, "price": 60 },
        { "_id": 4, "ticker": "ABC", "hour": 4, "price": 40 },
        { "_id": 5, "ticker": "XYZ", "hour": 1, "price": 30 },
        { "_id": 6, "ticker": "XYZ", "hour": 2, "price": 35 },
        { "_id": 7, "ticker": "XYZ", "hour": 3, "price": 25 }
    ])
    window_pipeline = [
        {
            "$setWindowFields": {
                "partitionBy": "$ticker",
                "sortBy": { "hour": 1 },
                "output": {
                    "lastPrice": {
                        "$last": "$price",
                        "window": { "documents": ["unbounded", "unbounded"] }
                    }
                }
            }
        }
    ]
    window_result = list(stock_prices.aggregate(window_pipeline))
    print('Window result:', window_result)

    client.close()

example()