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$size
该$size运算符用于返回数组字段中的项目数。这可以用来确定存储在文档中的数组中的元素数量。
参数
示例（MongoDB 外壳）
此示例说明如何使用$size运算符返回每个用户关注的团队数量。
创建示例文档
db.profiles.insertMany([
{ _id: 1, name: "John Doe", teams: ["Acme", "Widgets", "Gadgets"] },
{ _id: 2, name: "Jane Smith", teams: ["Acme", "Gadgets"] },
{ _id: 3, name: "Bob Johnson", teams: ["Acme", "Widgets", "Gadgets"] }
]);
查询示例
db.profiles.aggregate([
{
$project: {
_id: 0,
name: 1,
"numberOfTeams": { $size: "$teams" }
}
}
])
输出
[
{ name: 'John Doe', numberOfTeams: 3 },
{ name: 'Jane Smith', numberOfTeams: 2 },
{ name: 'Bob Johnson', numberOfTeams: 3 }
]
代码示例
要查看使用该$size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 main() {
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 profiles = db.collection('profiles');
const result = await profiles.aggregate([
{
$project: {
item: 1,
"numberOfTeams": { $size: "$teams" }
}
}
]).toArray();
console.log(result);
await client.close();
}
main();
- Python
-
from pymongo import MongoClient
def main():
client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
db = client['test']
profiles = db.profiles
result = list(profiles.aggregate([
{
'$project': {
'item': 1,
'numberOfTeams': { '$size': '$teams' }
}
}
]))
print(result)
client.close()
if __name__ == "__main__":
main()
