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# \$1project
<a name="project"></a>

Amazon DocumentDB 中的`$project`運算子可讓您選擇性地從輸出文件中包含或排除欄位、將值傳遞至下一個管道階段，以及從輸入文件值運算新欄位。

**參數**
+ `field`：要從輸出文件中包含或排除的欄位，可以是欄位路徑 （例如 "a.b.c")。
+ `1` 或 `true`：在輸出中包含 欄位。
+ `0` 或 `false`：從輸出中排除 欄位。

## 範例 (MongoDB Shell)
<a name="project-examples"></a>

下列範例示範在學生集合上使用 `$project` 運算子

**建立範例文件**

```
db.students.insertMany([
  { "_id": 1, "name": "Alejandro Rosalez", "math": 85, "science": 92, "grade": "A" },
  { "_id": 2, "name": "Carlos Salazar", "math": 78, "science": 84, "grade": "B" },
  { "_id": 3, "name": "Nikhil Jayashankar", "math": 95, "science": 89, "grade": "A" },
  { "_id": 4, "name": "Shirley Rodriguez", "math": 72, "science": 76, "grade": "B" }
  ]);
```

此查詢僅包含輸出中的 `name`和 `math` 欄位。除非明確排除，否則預設會包含 `_id` 欄位。

```
db.students.aggregate([
  { $project: { "name": 1, "math": 1 } }
])
```

**輸出**

```
{ _id: 1, name: "Alejandro Rosalez", math: 85 }
{ _id: 2, name: "Carlos Salazar", math: 78 }
{ _id: 3, name: "Nikhil Jayashankar", math: 95 }
{ _id: 4, name: "Shirley Rodriguez", math: 72 }
```

此查詢會從輸出中排除 `grade`和 `_id` 欄位，顯示所有其他欄位 (`name`、`math`、`science`)。

```
db.students.aggregate([
  { $project: { "grade": 0, "_id": 0 } }
])
```

**輸出**

```
{ name: "Alejandro Rosalez", math: 85, science: 92 }
{ name: "Carlos Salazar", math: 78, science: 84 }
{ name: "Nikhil Jayashankar", math: 95, science: 89 }
{ name: "Shirley Rodriguez", math: 72, science: 76 }
```

## 程式碼範例
<a name="project-code"></a>

若要檢視使用 `$project`命令的程式碼範例，請選擇您要使用的語言標籤：

------
#### [ 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('students');

  const result = await collection.aggregate([
    { $project: { "name": 1, "math": 1 } }
  ]).toArray();
  console.log(result);

  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['students']

    result = list(collection.aggregate([
        { '$project': { 'name': 1, 'math': 1 } }
    ]))
    print(result)

    client.close()

example()
```

------