# $arrayToObject (aggregation)

在本页面

* [定义](#definition)
* [行为](#behavior)
* [例子](#examples)

## 定义

**$arrayToObject**

*3.4.4版中的新功能。*

将数组转换为单个文档；数组必须为：

* 一个由两个元素组成的数组，其中第一个元素是字段名称，第二个元素是字段值：

  ```
  [ [ "item", "abc123"], [ "qty", 25 ] ]
  ```

-OR-

* 文件数组，包含两个字段，`k`并且`v` 其中：

  * 该`k`字段包含字段名称。
  * 该`v`字段包含该字段的值。

  ```
  [ { "k": "item", "v": "abc123"}, { "k": "qty", "v": 25 } ]
  ```

`$arrayToObject`具有以下语法：

```
{ $arrayToObject: <expression> }
```

`<expression>`可以是任何有效的表达解析为两个元件阵列或包含“k”和“V”域的文档阵列的阵列。

有关表达式的更多信息，请参见 表达式。

## 行为

如果字段名称在数组中重复，

* 从4.0.5开始，`$arrayToObject`使用该字段的最后一个值。对于4.0.0-4.0.4，使用的值取决于驱动程序。
* 从3.6.10开始，`$arrayToObject`使用该字段的最后一个值。对于3.6.0-3.6.9，使用的值取决于驱动程序。
* 从3.4.19开始，`$arrayToObject`使用该字段的最后一个值。对于3.4.0-3.4.19，使用的值取决于驱动程序。

| 例子                                                                                                   | 结果                                                                                             |
| ---------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------- |
| { $arrayToObject: { $literal: \[ { "k": "item", "v": "abc123"}, { "k": "qty", "v": 25 } ] } }        | { "item" : "abc123", "qty" : 25 }                                                              |
| { $arrayToObject: { $literal: \[ \[ "item", "abc123"], \[ "qty", 25 ] ] } }                          | { "item" : "abc123", "qty" : 25 }                                                              |
| { $arrayToObject: { $literal: \[ { "k": "item", "v": "123abc"}, { "k": "item", "v": "abc123" } ] } } | { "item" : "abc123" } 从版本4.0.5+（3.6.10+和3.4.19+）开始，如果字段名称在数组中重复，则`$arrayToObject` 使用该字段的最后一个值。 |

## 例子

### `$arrayToObject` 例子

考虑`inventory`包含以下文档的集合：

```
{ "_id" : 1, "item" : "ABC1",  dimensions: [ { "k": "l", "v": 25} , { "k": "w", "v": 10 }, { "k": "uom", "v": "cm" } ] }
{ "_id" : 2, "item" : "ABC2",  dimensions: [ [ "l", 50 ], [ "w",  25 ], [ "uom", "cm" ] ] }
{ "_id" : 3, "item" : "ABC3",  dimensions: [ [ "l", 25 ], [ "l",  "cm" ], [ "l", 50 ] ] }
```

以下聚合管道操作使用 `$arrayToObject`将该`dimensions`字段作为文档返回：

```
db.inventory.aggregate(
   [
      {
         $project: {
            item: 1,
            dimensions: { $arrayToObject: "$dimensions" }
         }
      }
   ]
)
```

该操作返回以下结果：

```
{ "_id" : 1, "item" : "ABC1", "dimensions" : { "l" : 25, "w" : 10, "uom" : "cm" } }
{ "_id" : 2, "item" : "ABC2", "dimensions" : { "l" : 50, "w" : 25, "uom" : "cm" } }
{ "_id" : 3, "item" : "ABC3", "dimensions" : { "l" : 50 } }
```

从版本4.0.5+（3.6.10+和3.4.19+）开始，如果字段名称在数组中重复，则`$arrayToObject`使用该字段的最后一个值。

### `$objectToArray`+ `$arrayToObject`示例

考虑`inventory`包含以下文档的集合：

```
{ "_id" : 1, "item" : "ABC1", instock: { warehouse1: 2500, warehouse2: 500 } }
{ "_id" : 2, "item" : "ABC2", instock: { warehouse2: 500, warehouse3: 200} }
```

以下聚合管道操作将计算每个物料的总存货并将其添加到`instock`凭证中：

```
db.inventory.aggregate( [
   { $addFields: { instock: { $objectToArray: "$instock" } } },
   { $addFields: { instock: { $concatArrays: [ "$instock", [ { "k": "total", "v": { $sum: "$instock.v" } } ] ] } } } ,
   { $addFields: { instock: { $arrayToObject: "$instock" } } }
] )
```

该操作返回以下内容：

```
{ "_id" : 1, "item" : "ABC1", "instock" : { "warehouse1" : 2500, "warehouse2" : 500, "total" : 3000 } }
{ "_id" : 2, "item" : "ABC2", "instock" : { "warehouse2" : 500, "warehouse3" : 200, "total" : 700 } }
```

> 也可以看看
>
> `$objectToArray`

译者：李冠飞

校对：


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.mongoing.com/can-kao/yun-suan-fu/aggregation-pipeline-operators/arraytoobject-aggregation.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.