$concatArrays (聚合)

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定义

行为
示例

定义

$concatArrays

连接数组，返回连接后的数组。

$concatArrays具有以下语法

{ $concatArrays: [ <array1>, <array2>, ... ] }

《<array>》表达式可以是任何有效的表达式，只要它们解析为数组即可。有关表达式的更多信息，请参阅表达式运算符。

如果任何参数解析为null值或引用了缺失的字段，则$concatArrays返回null。

行为

示例

结果

{ $concatArrays: [
   [ "hello", " "], [ "world" ]
] }

[ "hello", " ", "world" ]

{ $concatArrays: [
   [ "hello", " "],
   [ [ "world" ], "again"]
] }

[ "hello", " ", [ "world" ], "again" ]

示例

创建一个名为warehouses的集合，包含以下文档

db.warehouses.insertMany( [
   { _id : 1, instock: [ "chocolate" ], ordered: [ "butter", "apples" ] },
   { _id : 2, instock: [ "apples", "pudding", "pie" ] },
   { _id : 3, instock: [ "pears", "pecans" ], ordered: [ "cherries" ] },
   { _id : 4, instock: [ "ice cream" ], ordered: [ ] }
] )

以下示例将instock和ordered数组连接起来

db.warehouses.aggregate( [
   { $project: { items: { $concatArrays: [ "$instock", "$ordered" ] } } }
] )

{ _id : 1, items : [ "chocolate", "butter", "apples" ] }
{ _id : 2, items : null }
{ _id : 3, items : [ "pears", "pecans", "cherries" ] }
{ _id : 4, items : [ "ice cream" ] }

提示

另请参阅

$push

$concat

$cond