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$ltrim
4.0 版的新增内容。
弹性集群不支持。
Amazon DocumentDB 中的$ltrim运算符用于从字符串中删除前导字符。默认情况下，它会删除前导空格字符，但您也可以通过传递 chars 参数来指定一组要删除的字符。
参数
-
input：要从中删除前导空格字符的输入字符串。
-
chars:（可选）删除特定字符。
示例（MongoDB 外壳）
以下示例演示了从字符串开头删除指定字符 (” *”) 的用法。$ltrim
创建示例文档
db.collection.insertMany([
{ name: " *John Doe", age: 30 },
{ name: "Jane Doe*", age: 25 },
{ name: " Bob Smith ", age: 35 }
]);
查询示例
db.collection.aggregate([
{
$project: {
_id: 0,
name: {
$ltrim: { input: "$name", chars: " *" }
},
age: 1
}
}
]);
输出
[
{ "name": "John Doe", "age": 30 },
{ "name": "Jane Doe ", "age": 25 },
{ "name": "Bob Smith ", "age": 35 }
]
代码示例
要查看使用该$ltrim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 example() {
const client = await MongoClient.connect('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');
const collection = db.collection('collection');
const pipeline = [
{
$project: {
_id: 0,
name: {
$ltrim: {
input: '$name',
chars: ' *'
}
},
age: 1
}
}
];
const result = await collection.aggregate(pipeline).toArray();
console.dir(result, { depth: null });
} finally {
await client.close();
}
}
example().catch(console.error);
- Python
-
from pymongo import MongoClient
def example():
try:
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.collection
pipeline = [
{
"$project": {
"_id": 0,
"name": {
"$ltrim": {
"input": "$name",
"chars": " *"
}
},
"age": 1
}
}
]
results = collection.aggregate(pipeline)
for doc in results:
print(doc)
except Exception as e:
print(f"An error occurred: {e}")
finally:
client.close()
example()
