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$slice
$slice投影运算符限制查询结果中返回的数组元素的数量。它允许您从数组字段的开头或结尾检索特定数量的元素，而无需加载整个数组。
参数
示例（MongoDB 外壳）
以下示例演示如何使用$slice投影运算符仅返回数组字段中的前两项。
创建示例文档
db.inventory.insertMany([
{ _id: 1, item: "notebook", tags: ["office", "school", "supplies", "writing"] },
{ _id: 2, item: "pen", tags: ["office", "writing"] },
{ _id: 3, item: "folder", tags: ["office", "supplies", "storage", "organization"] }
]);
查询示例
db.inventory.find(
{},
{ item: 1, tags: { $slice: 2 } }
)
输出
{ "_id" : 1, "item" : "notebook", "tags" : [ "office", "school" ] }
{ "_id" : 2, "item" : "pen", "tags" : [ "office", "writing" ] }
{ "_id" : 3, "item" : "folder", "tags" : [ "office", "supplies" ] }
代码示例
要查看使用$slice投影运算符的代码示例，请选择要使用的语言的选项卡：
- Node.js
-
const { MongoClient } = require('mongodb');
async function 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');
const collection = db.collection('inventory');
const result = await collection.find(
{},
{ projection: { item: 1, tags: { $slice: 2 } } }
).toArray();
console.log(JSON.stringify(result, null, 2));
await client.close();
}
example();
- Python
-
from pymongo import MongoClient
def example():
client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
db = client['test']
collection = db['inventory']
result = list(collection.find(
{},
{'item': 1, 'tags': {'$slice': 2}}
))
print(result)
client.close()
example()
