本文属于机器翻译版本。若本译文内容与英语原文存在差异，则一律以英文原文为准。
$firstN
8.0.1 版中的新增内容。
亚马逊 DocumentDB 中的$firstN运算符返回前 N 个元素。当在$group阶段中用作累加器时，它返回每组中前 N 个值的数组。当用作数组表达式运算符时，它返回数组的前 N 个元素。
参数
示例（MongoDB Shell）
以下示例说明如何在聚合期间使用$firstN累加器检索每个项目的前两个数量。
$firstN按照文档到达$group阶段的顺序选择值。要返回特定排序（例如，按日期或分数）的前 N 个值，请在前面添加一个$sort阶段$group。
创建示例文档
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") }
]);
查询示例
db.sales.aggregate([
{ $group: { _id: "$item", firstTwoQuantities: { $firstN: { input: "$quantity", n: 2 } } } }
]);
输出
[
{ "_id": "abc", "firstTwoQuantities": [10, 5] },
{ "_id": "xyz", "firstTwoQuantities": [15, 7] }
]
表达式用法示例（MongoDB Shell）
该$firstN运算符也可以用作$project阶段内的表达式，以返回数组字段的前 N 个元素。
创建示例文档
db.inventory.insertMany([
{ _id: 1, item: "abc", tags: ["red", "green", "blue", "yellow", "purple"] },
{ _id: 2, item: "xyz", tags: ["alpha", "beta", "gamma"] }
]);
查询示例
db.inventory.aggregate([
{ $project: {
firstThreeTags: { $firstN: { input: "$tags", n: 3 } }
}}
]);
输出
[
{ "_id": 1, "firstThreeTags": ["red", "green", "blue"] },
{ "_id": 2, "firstThreeTags": ["alpha", "beta", "gamma"] }
]
窗口运算符用法示例（MongoDB Shell）
8.0.2 版中的新内容。
该$firstN操作员还可以在$setWindowFields舞台中用作窗口操作员。在这种情况下，它返回一个包含每个窗口中文档的第一个n值的数组。您可以在output字段下指定运算符，也可以选择使用window文档定义窗口边界。
在中用作窗口运算符时$setWindowFields，中间数据限制$firstN为 100 MB。超过此限制的操作会返回错误。
创建示例文档
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 }
]);
查询示例
以下示例对文档进行分区ticker，按分区排序hour，并返回从分区开始到当前文档的前两个价格。
db.stockPrices.aggregate([
{
$setWindowFields: {
partitionBy: "$ticker",
sortBy: { hour: 1 },
output: {
firstTwoSoFar: {
$firstN: { input: "$price", n: 2 },
window: { documents: ["unbounded", "current"] }
}
}
}
}
]);
输出
[
{ "_id": 1, "ticker": "ABC", "hour": 1, "price": 50, "firstTwoSoFar": [50] },
{ "_id": 2, "ticker": "ABC", "hour": 2, "price": 45, "firstTwoSoFar": [50, 45] },
{ "_id": 3, "ticker": "ABC", "hour": 3, "price": 60, "firstTwoSoFar": [50, 45] },
{ "_id": 4, "ticker": "ABC", "hour": 4, "price": 40, "firstTwoSoFar": [50, 45] },
{ "_id": 5, "ticker": "XYZ", "hour": 1, "price": 30, "firstTwoSoFar": [30] },
{ "_id": 6, "ticker": "XYZ", "hour": 2, "price": 35, "firstTwoSoFar": [30, 35] },
{ "_id": 7, "ticker": "XYZ", "hour": 3, "price": 25, "firstTwoSoFar": [30, 35] }
]
每个文档都使用firstTwoSoFar其窗口中的前两个price值（hour按顺序排列）进行扩展。
代码示例
要查看使用$firstN运算符的代码示例，请选择要使用的语言的选项卡。以下示例显示累加器的用法 (in$group)、表达式的用法 (in$project) 和窗口运算符的用法 (in$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: first 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", firstTwoQuantities: { $firstN: { input: "$quantity", n: 2 } } } }
]).toArray();
console.log('Accumulator result:', accumulatorResult);
// Expression usage: first 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: { firstThreeTags: { $firstN: { input: "$tags", n: 3 } } } }
]).toArray();
console.log('Expression result:', expressionResult);
// Window operator usage: first 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: {
firstTwoSoFar: {
$firstN: { input: "$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: first 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', 'firstTwoQuantities': { '$firstN': { 'input': '$quantity', 'n': 2 } } } }
]))
print('Accumulator result:', accumulator_result)
# Expression usage: first 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([
{ '$project': { 'firstThreeTags': { '$firstN': { 'input': '$tags', 'n': 3 } } } }
]))
print('Expression result:', expression_result)
# Window operator usage: first N values over a window in each partition
stock_prices = db['stockPrices']
stock_prices.insert_many([
{ '_id': 1, 'ticker': 'ABC', 'hour': 1, 'price': 50 },
{ '_id': 2, 'ticker': 'ABC', 'hour': 2, 'price': 45 },
{ '_id': 3, 'ticker': 'ABC', 'hour': 3, 'price': 60 },
{ '_id': 4, 'ticker': 'ABC', 'hour': 4, 'price': 40 },
{ '_id': 5, 'ticker': 'XYZ', 'hour': 1, 'price': 30 },
{ '_id': 6, 'ticker': 'XYZ', 'hour': 2, 'price': 35 },
{ '_id': 7, 'ticker': 'XYZ', 'hour': 3, 'price': 25 }
])
window_result = list(stock_prices.aggregate([
{
'$setWindowFields': {
'partitionBy': '$ticker',
'sortBy': { 'hour': 1 },
'output': {
'firstTwoSoFar': {
'$firstN': { 'input': '$price', 'n': 2 },
'window': { 'documents': ['unbounded', 'current'] }
}
}
}
}
]))
print('Window result:', window_result)
finally:
client.close()
example()
