$last

L'$lastopérateur d'Amazon DocumentDB est utilisé pour renvoyer le dernier élément d'un tableau qui correspond aux critères de requête. Il est particulièrement utile pour récupérer le plus récent ou le dernier élément d'un tableau qui satisfait une condition spécifique.

Paramètres

• expression: expression qui correspond aux éléments du tableau.

Exemple (MongoDB Shell)

L'exemple suivant montre l'utilisation de l'$lastopérateur en combinaison avec $filter pour récupérer le dernier élément d'un tableau qui répond à une condition spécifique (par exemple, le sujet est « science »).

Créez des exemples de documents

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 }
    ]
  }
]);

Exemple de requête

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

Sortie

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

Exemple d'utilisation d'un opérateur de fenêtre (MongoDB Shell)

Nouveau depuis la version 8.0.2.

L'$lastopérateur peut également être utilisé comme opérateur de fenêtre dans la $setWindowFields scène. Dans ce contexte, il renvoie la valeur expression du dernier document de chaque fenêtre. Vous spécifiez l'opérateur dans le output champ et définissez éventuellement les limites de la fenêtre à l'aide d'un window document.

Créez des exemples de documents

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 }
]);

Exemple de requête

L'exemple suivant partitionne les documents particker, trie chaque partition par hour et renvoie le dernier (dernier) prix enregistré dans chaque partition.

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

Sortie

[
  { "_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 }
]

Chaque document est augmenté du prix du dernier document de sa fenêtre (le dernier hour de la partition). lastPrice

Exemples de code

Pour afficher un exemple de code d'utilisation de l'$lastopérateur, choisissez l'onglet correspondant à la langue que vous souhaitez utiliser. Les exemples suivants montrent à la fois l'utilisation des expressions (in$project) et l'utilisation des opérateurs de fenêtre (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()