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$out
Amazon DocumentDB 中的$out运算符用于将聚合管道的结果写入指定的集合。
$out应该是最后一步。
参数
注意：如果集合已经存在，则会将其替换为聚合阶段的结果。
示例（MongoDB 外壳）
以下示例演示如何在 Amazon DocumentDB 中使用$out运算符将聚合管道的结果写入新集合。
创建示例文档
db.products.insertMany([
{ _id: 1, name: "Wireless Headphones", category: "Electronics", price: 100.0 },
{ _id: 2, name: "Smartphone", category: "Electronics", price: 200.0 },
{ _id: 3, name: "JavaScript Guide", category: "Books", price: 50.0 },
{ _id: 4, name: "Database Design Handbook", category: "Books", price: 75.0 }
]);
查询示例
db.products.aggregate([
{ $group: { _id: "$category", totalPrice: { $sum: "$price" } } },
{ $out: "product_categories" }
])
输出
无（结果写入输出集合）。
聚合管道按类别对产品进行分组，并计算每个类别的商品总价。$out操作员将结果写入名为 “product_categories” 的新集合。
要查看输出集合中的结果，请执行以下操作：
db.product_categories.find()
[
{ "_id" : "Books", "totalPrice" : 125 },
{ "_id" : "Electronics", "totalPrice" : 300 }
]
代码示例
要查看使用该$out命令的代码示例，请选择要使用的语言的选项卡：
- Node.js
-
const { MongoClient } = require('mongodb');
async function demo_out_operator() {
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 products = db.collection('products');
// Execute aggregation with $out - results are stored in 'product_categories' collection
await products.aggregate([
{ $group: { _id: "$category", totalPrice: { $sum: "$price" } } },
{ $out: "product_categories" }
]).toArray();
// Retrieve the results from the output collection (limited to 20 records)
const productCategories = db.collection('product_categories');
const results = await productCategories.find({}).limit(20).toArray();
console.log('Results stored in product_categories collection:', results);
await client.close();
}
demo_out_operator();
- Python
-
from pymongo import MongoClient
def demo_out_operator():
client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
db = client['test']
products = db['products']
# Execute aggregation with $out - results are stored in 'product_categories' collection
list(products.aggregate([
{ '$group': { '_id': '$category', 'totalPrice': { '$sum': '$price' } } },
{ '$out': 'product_categories' }
]))
# Retrieve the results from the output collection (limited to 20 records)
product_categories = db['product_categories']
results = list(product_categories.find({}).limit(20))
print('Results stored in product_categories collection:', results)
client.close()
demo_out_operator()
