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$$KEEP
$$KEEP系统变量与聚合管道中的$redact阶段一起使用，以保持当前文档或字段不变，并将其包含在输出中。
参数
无
示例（MongoDB 外壳）
以下示例演示了在 Amazon DocumentDB 聚合管道$$KEEP中的用法。只有当访问权限等于 “公开” 时，才会保留文档，否则它们将被删除。
创建示例文档
db.articles.insertMany([
{ title: "Article A", access: "public", content: "Visible content" },
{ title: "Article B", access: "private", content: "Hidden content" }
]);
查询示例
db.articles.aggregate([
{
$redact: {
$cond: [
{ $eq: ["$access", "public"] },
"$$KEEP",
"$$PRUNE"
]
}
}
]);
输出
[
{
"_id" : ObjectId("..."),
"title" : "Article A",
"access" : "public",
"content" : "Visible content"
}
]
代码示例
要查看使用该$$KEEP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 run() {
const client = new MongoClient(
'mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0'
);
try {
await client.connect();
const db = client.db('test');
const articles = db.collection('articles');
const pipeline = [
{
$redact: {
$cond: [
{ $eq: ["$access", "public"] },
"$$KEEP",
"$$PRUNE"
]
}
}
];
const results = await articles.aggregate(pipeline).toArray();
console.log(results);
} finally {
await client.close();
}
}
run().catch(console.error);
- Python
-
from pymongo import MongoClient
client = MongoClient(
"mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0"
)
db = client.test
articles = db.articles
pipeline = [
{
"$redact": {
"$cond": [
{"$eq": ["$access", "public"]},
"$$KEEP",
"$$PRUNE"
]
}
}
]
results = list(articles.aggregate(pipeline))
print(results)
client.close()
