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。
$lastN
8.0.1 版的新功能。
Amazon DocumentDB 中的$lastN運算子會傳回最後一個 N 元素。在$group階段中用作累積器時，它會傳回每個群組中最後一個 N 值的陣列。用作陣列表達式運算子時，它會傳回陣列的最後 N 個元素。
參數
範例 (MongoDB Shell)
下列範例示範如何在彙總期間使用$lastN累積器擷取每個項目的最後兩個數量。
$lastN 會依文件到達$group階段的順序選取值。若要傳回特定排序的最後一個 N 值 （例如，依日期或分數），請在 之前新增$sort階段$group。
建立範例文件
db.sales.insertMany([
{ item: "abc", quantity: 10, date: ISODate("2023-01-01") },
{ item: "abc", quantity: 5, date: ISODate("2023-01-02") },
{ item: "abc", quantity: 8, date: ISODate("2023-01-03") },
{ item: "xyz", quantity: 15, date: ISODate("2023-01-01") },
{ item: "xyz", quantity: 7, date: ISODate("2023-01-02") },
{ item: "xyz", quantity: 3, date: ISODate("2023-01-03") }
]);
查詢範例
db.sales.aggregate([
{ $group: { _id: "$item", lastTwoQuantities: { $lastN: { input: "$quantity", n: 2 } } } }
]);
輸出
[
{ "_id": "abc", "lastTwoQuantities": [5, 8] },
{ "_id": "xyz", "lastTwoQuantities": [7, 3] }
]
表達式用量範例 (MongoDB Shell)
運算$lastN子也可以用作$project階段內的表達式，以傳回陣列欄位的最後 N 個元素。
建立範例文件
db.inventory.insertMany([
{ _id: 1, item: "abc", tags: ["red", "green", "blue", "yellow", "purple"] },
{ _id: 2, item: "xyz", tags: ["alpha", "beta", "gamma"] }
]);
查詢範例
db.inventory.aggregate([
{ $project: {
lastThreeTags: { $lastN: { input: "$tags", n: 3 } }
}}
]);
輸出
[
{ "_id": 1, "lastThreeTags": ["blue", "yellow", "purple"] },
{ "_id": 2, "lastThreeTags": ["alpha", "beta", "gamma"] }
]
視窗運算子用量範例 (MongoDB Shell)
8.0.2 版的新功能。
該$lastN運算子也可以用作$setWindowFields階段中的視窗運算子。在這種情況下，它會傳回每個視窗中文件最多最後一個n值的陣列。您可以在 output 欄位下指定運算子，並選擇性地使用window文件定義視窗邊界。
在 中用作視窗運算子時$setWindowFields， 限制$lastN為 100 MB 的中繼資料。超過此限制的操作會傳回錯誤。
建立範例文件
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 }
]);
查詢範例
下列範例會依 分割文件ticker、依 排序每個分割區hour，並傳回從分割區開頭到目前文件的兩個最新價格。
db.stockPrices.aggregate([
{
$setWindowFields: {
partitionBy: "$ticker",
sortBy: { hour: 1 },
output: {
lastTwoSoFar: {
$lastN: { input: "$price", n: 2 },
window: { documents: ["unbounded", "current"] }
}
}
}
}
]);
輸出
[
{ "_id": 1, "ticker": "ABC", "hour": 1, "price": 50, "lastTwoSoFar": [50] },
{ "_id": 2, "ticker": "ABC", "hour": 2, "price": 45, "lastTwoSoFar": [50, 45] },
{ "_id": 3, "ticker": "ABC", "hour": 3, "price": 60, "lastTwoSoFar": [45, 60] },
{ "_id": 4, "ticker": "ABC", "hour": 4, "price": 40, "lastTwoSoFar": [60, 40] },
{ "_id": 5, "ticker": "XYZ", "hour": 1, "price": 30, "lastTwoSoFar": [30] },
{ "_id": 6, "ticker": "XYZ", "hour": 2, "price": 35, "lastTwoSoFar": [30, 35] },
{ "_id": 7, "ticker": "XYZ", "hour": 3, "price": 25, "lastTwoSoFar": [35, 25] }
]
每個文件都會以 擴增lastTwoSoFar，也就是其視窗內的最後兩個price值 (hour依序）。
程式碼範例
若要檢視使用 $lastN 運算子的程式碼範例，請選擇您要使用的語言標籤。下列範例顯示累積器用量 （在 中$group)、表達式用量 （在 中$project) 和視窗運算子用量 （在 中$setWindowFields)：
- Node.js
-
const { MongoClient } = require('mongodb');
async function example() {
const client = new MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');
try {
await client.connect();
const db = client.db('test');
// Accumulator usage: last N values per group
const sales = db.collection('sales');
await sales.insertMany([
{ item: "abc", quantity: 10, date: new Date("2023-01-01") },
{ item: "abc", quantity: 5, date: new Date("2023-01-02") },
{ item: "abc", quantity: 8, date: new Date("2023-01-03") },
{ item: "xyz", quantity: 15, date: new Date("2023-01-01") },
{ item: "xyz", quantity: 7, date: new Date("2023-01-02") },
{ item: "xyz", quantity: 3, date: new Date("2023-01-03") }
]);
const accumulatorResult = await sales.aggregate([
{ $group: { _id: "$item", lastTwoQuantities: { $lastN: { input: "$quantity", n: 2 } } } }
]).toArray();
console.log('Accumulator result:', accumulatorResult);
// Expression usage: last N elements of an array field
const inventory = db.collection('inventory');
await inventory.insertMany([
{ _id: 1, item: "abc", tags: ["red", "green", "blue", "yellow", "purple"] },
{ _id: 2, item: "xyz", tags: ["alpha", "beta", "gamma"] }
]);
const expressionResult = await inventory.aggregate([
{ $project: { lastThreeTags: { $lastN: { input: "$tags", n: 3 } } } }
]).toArray();
console.log('Expression result:', expressionResult);
// Window operator usage: last N values over a window in 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: {
lastTwoSoFar: {
$lastN: { input: "$price", n: 2 },
window: { documents: ["unbounded", "current"] }
}
}
}
}
]).toArray();
console.log('Window result:', windowResult);
} finally {
await client.close();
}
}
example();
- Python
-
from pymongo import MongoClient
from datetime import datetime
def example():
client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
try:
db = client['test']
# Accumulator usage: last N values per group
sales = db['sales']
sales.insert_many([
{ 'item': 'abc', 'quantity': 10, 'date': datetime(2023, 1, 1) },
{ 'item': 'abc', 'quantity': 5, 'date': datetime(2023, 1, 2) },
{ 'item': 'abc', 'quantity': 8, 'date': datetime(2023, 1, 3) },
{ 'item': 'xyz', 'quantity': 15, 'date': datetime(2023, 1, 1) },
{ 'item': 'xyz', 'quantity': 7, 'date': datetime(2023, 1, 2) },
{ 'item': 'xyz', 'quantity': 3, 'date': datetime(2023, 1, 3) }
])
accumulator_result = list(sales.aggregate([
{ '$group': { '_id': '$item', 'lastTwoQuantities': { '$lastN': { 'input': '$quantity', 'n': 2 } } } }
]))
print('Accumulator result:', accumulator_result)
# Expression usage: last N elements of an array field
inventory = db['inventory']
inventory.insert_many([
{ '_id': 1, 'item': 'abc', 'tags': ['red', 'green', 'blue', 'yellow', 'purple'] },
{ '_id': 2, 'item': 'xyz', 'tags': ['alpha', 'beta', 'gamma'] }
])
expression_result = list(inventory.aggregate([
{ '$project': { 'lastThreeTags': { '$lastN': { 'input': '$tags', 'n': 3 } } } }
]))
print('Expression result:', expression_result)
# Window operator usage: last N values over a window in each partition
stock_prices = db['stockPrices']
stock_prices.insert_many([
{ '_id': 1, 'ticker': 'ABC', 'hour': 1, 'price': 50 },
{ '_id': 2, 'ticker': 'ABC', 'hour': 2, 'price': 45 },
{ '_id': 3, 'ticker': 'ABC', 'hour': 3, 'price': 60 },
{ '_id': 4, 'ticker': 'ABC', 'hour': 4, 'price': 40 },
{ '_id': 5, 'ticker': 'XYZ', 'hour': 1, 'price': 30 },
{ '_id': 6, 'ticker': 'XYZ', 'hour': 2, 'price': 35 },
{ '_id': 7, 'ticker': 'XYZ', 'hour': 3, 'price': 25 }
])
window_result = list(stock_prices.aggregate([
{
'$setWindowFields': {
'partitionBy': '$ticker',
'sortBy': { 'hour': 1 },
'output': {
'lastTwoSoFar': {
'$lastN': { 'input': '$price', 'n': 2 },
'window': { 'documents': ['unbounded', 'current'] }
}
}
}
}
]))
print('Window result:', window_result)
finally:
client.close()
example()
