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

The `$addFields` stage in the Amazon DocumentDB aggregation pipeline allows you to add new computed fields to documents. This can be useful for adding derived or transformed data to the documents.

**Parameters**
+ `newField`: The name of the new field to add.
+ `expression`: An expression that resolves to the value of the new field.

## Example (MongoDB Shell)
<a name="addFields-examples"></a>

The following example demonstrates how to use `$addFields` to add a new field `TotalInventory` that calculates the total inventory based on the `Inventory.OnHand` and `Inventory.OrderQnty` fields.

**Create sample documents**

```
db.example.insertMany([
  {
    "Item": "Spray Paint",
    "Colors": ["Black", "Red", "Green", "Blue"],
    "Inventory": {
      "OnHand": 47,
      "MinOnHand": 50,
      "OrderQnty": 36
    },
    "UnitPrice": 3.99
  },
  {
    "Item": "Ruler",
    "Colors": ["Red", "Green", "Blue", "Clear", "Yellow"],
    "Inventory": {
      "OnHand": 47,
      "MinOnHand": 40
    },
    "UnitPrice": 0.89
  }
]);
```

**Query example**

```
db.example.aggregate([
  {
    $addFields: {
      TotalInventory: { $add: ["$Inventory.OnHand", "$Inventory.OrderQnty"] }
    }
  }
])
```

**Output**

```
[
  {
    "_id" : ObjectId("5bedafbcf65ff161707de24f"),
    "Item" : "Ruler",
    "Colors" : [ "Red", "Green", "Blue", "Clear", "Yellow" ],
    "Inventory" : {
      "OnHand" : 47,
      "MinOnHand" : 40
    },
    "UnitPrice" : 0.89,
    "TotalInventory" : 47
  },
  {
    "_id" : ObjectId("5bedafbcf65ff161707de250"),
    "Item" : "Spray Paint",
    "Colors" : [ "Black", "Red", "Green", "Blue" ],
    "Inventory" : {
      "OnHand" : 47,
      "MinOnHand" : 50,
      "OrderQnty" : 36
    },
    "UnitPrice" : 3.99,
    "TotalInventory" : 83
  }
]
```

## Code examples
<a name="addFields-code"></a>

To view a code example for using the `$addFields` command, choose the tab for the language that you want to use:

------
#### [ Node.js ]

```
const { MongoClient } = require('mongodb');

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('example');

const result = await collection.aggregate([
  {
    $addFields: {
      TotalInventory: { $add: ['$Inventory.OnHand', '$Inventory.OrderQnty'] }
    }
  }
]).toArray();

console.log(result);

await client.close();
```

------
#### [ Python ]

```
from pymongo import MongoClient

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['example']

result = list(collection.aggregate([
    {
        '$addFields': {
            'TotalInventory': { '$add': ['$Inventory.OnHand', '$Inventory.OrderQnty'] }
        }
    }
]))

print(result)
client.close()
```

------