Database Manual / Reference / Query Language / Expressions

$arrayToObject (expression operator)（表达式运算符）

Definition定义

$arrayToObject

Converts an array into a single document.将数组转换为单个文档。

$arrayToObject has the following syntax:具有以下语法：

{ $arrayToObject: <expression> }

The <expression> can be any valid expression that resolves to:<expression>可以是解析为以下内容的任何有效表达式：

  • An array of two-element arrays where the first element is the field name, and the second element is the field value:一个由两个元素组成的数组，其中第一个元素是字段名，第二个元素是域值：

    $arrayToObject: [
        [ [ "item", "abc123" ], [ "qty", 25 ] ]
    ]

- OR -

  • An array of documents that contains two fields, k and v where:包含两个字段k和v的文档数组，其中：

    • The k field contains the field name.k字段包含字段名。
    • The v field contains the value of the field.v字段包含字段的值。
    $arrayToObject: [
       [
          { "k": "item", "v": "abc123" },
          { "k": "qty", "v": 25 }
       ]
    ]

Behavior行为

If you pass in an empty array to $arrayToObject, MongoDB creates an empty object.如果你向$arrayToObject传递一个空数组，MongoDB会创建一个空对象。

If the name of a field repeats in the array,如果字段的名称在数组中重复，

  • Starting in 4.0.5, $arrayToObject uses the last value for that field. For 4.0.0-4.0.4, the value used depends on the driver.从4.0.5开始，$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。
  • Starting in 3.6.10, $arrayToObject uses the last value for that field. For 3.6.0-3.6.9, the value used depends on the driver.从3.6.10开始，$arrayToObject使用该字段的最后一个值。对于3.6.0-3.6.9，使用的值取决于驾驶员。
  • Starting in 3.4.19, $arrayToObject uses the last value for that field. For 3.4.0-3.4.19, the value uses depends on the driver.从3.4.19开始，$arrayToObject使用该字段的最后一个值。对于3.4.0-3.4.19，使用的值取决于驾驶员。
Example示例Results结果
 
{ $arrayToObject: [
   [ { "k": "item", "v": "abc123" },
     { "k": "qty", "v": "$qty" } ]
] }

Given a document with the field qty and value 25, $arrayToObject returns:给定一个字段为qty、值为25的文档，$arrayToObject返回：

{ "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" }

Starting in versions 4.0.5+ (3.6.10+ and 3.4.19+), if the name of a field repeats in the array, $arrayToObject uses the last value for that field.从4.0.5+版本（3.6.10+和3.4.19+）开始，如果某个字段的名称在数组中重复，$arrayToObject将使用该字段的最后一个值。

Examples示例

$arrayToObject Example示例

Consider a inventory collection with the following documents:考虑使用以下文件进行inventory集合：

db.inventory.insertMany( [
   { _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 ] ] }
] )

The following aggregation pipeline operation use the $arrayToObject to return the dimensions field as a document:以下聚合管道操作使用$arrayToObject将维度字段作为文档返回：

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

The operation returns the following:该操作返回以下内容：

{ _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 } }

Starting in versions 4.0.5+ (3.6.10+ and 3.4.19+), if the name of a field repeats in the array, $arrayToObject uses the last value for that field.从4.0.5+版本（3.6.10+和3.4.19+）开始，如果某个字段的名称在数组中重复，$arrayToObject将使用该字段的最后一个值。

$objectToArray + $arrayToObject Example示例

Consider an inventory collection with the following documents:考虑使用以下文件进行inventory集合：

db.inventory.insertMany( [
   { _id: 1, item: "ABC1", instock: { warehouse1: 2500, warehouse2: 500 } },
   { _id: 2, item: "ABC2", instock: { warehouse2: 500, warehouse3: 200},
   }
] )

The following aggregation pipeline operation calculates the total in stock for each item and adds to the instock document:以下聚合管道操作计算每个项目的总库存，并添加到instock文档中：

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

The operation returns the following: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 } }

Tip

$objectToArray