$objectToArray聚合运算符用于将文档转换为数组，返回的数组的每个元素是一个文档，包含原始文档中的一个字段名和字段值，k的值为字段名，v的值为字段值。

语法

{ $objectToArray: <object> }

<object>可以用是任何能够解析为文档的表达式，$objectToArray只处理文档的顶层字段，如果<object>文档有内嵌文档，$objectToArray不会进行递归处理内嵌文档的字段。

使用

举例来说明使用规则：

例1，无内嵌文档的情况

{ $objectToArray: { item: "foo", qty: 25 } }

结果：

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

例2，有内嵌文档的情况

{ $objectToArray: {item: "foo",qty: 25,size: { len: 25, w: 10, uom: "cm" }} }

结果：

[{"k" : "item","v" : "foo"},{"k" : "qty","v" : 25},{"k" : "size","v" : {"len" : 25,"w" : 10,"uom" : "cm"}}
]

举例

objectToAarray 举例

inventory集合有下列文档：

{ "_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" : "XYZ1",  dimensions: { l: 70, w: 75, uom: "cm" } }

下面的聚合操作使用$objectToArray运算符将dimensions作为数组返回：

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

操作返回下面的结果：

{ "_id" : 1, "item" : "ABC1", "dimensions" : [ { "k" : "l", "v" : 25 }, { "k" : "w", "v" : 10 }, { "k" : "uom", "v" : "cm" } ] }
{ "_id" : 2, "item" : "ABC2", "dimensions" : [ { "k" : "l", "v" : 50 }, { "k" : "w", "v" : 25 }, { "k" : "uom", "v" : "cm" } ] }
{ "_id" : 3, "item" : "XYZ1", "dimensions" : [ { "k" : "l", "v" : 70 }, { "k" : "w", "v" : 75 }, { "k" : "uom", "v" : "cm" } ] }

使用objectToArray对内嵌字段求和

inventory集合有下列文档：

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

下面的聚合操作使用$objectToArray和$unwind以及$group一起计算每个仓库的库存总数：

db.inventory.aggregate([{ $project: { warehouses: { $objectToArray: "$instock" } } },{ $unwind: "$warehouses" },{ $group: { _id: "$warehouses.k", total: { $sum: "$warehouses.v" } } }
])

操作返回下面的结果：

{ "_id" : "warehouse3", "total" : 200 }
{ "_id" : "warehouse2", "total" : 1000 }
{ "_id" : "warehouse1", "total" : 2500 }

$objectToArray与$arrayToObject一起使用的例子

inventory集合有下列文档：

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

下面的聚合操作计算每个商品的库存总额并添加到库存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" } } }
] )

操作返回下面的结果：

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