$size (aggregation)

在本页面

2.6 版的新功能。

Definition

$size具有以下语法：

{ $size: <expression> }

$size的参数可以是任何expression，只要它解析为数组即可。有关表达式的更多信息，请参见Expressions。

Behavior

$size的参数必须解析为数组。如果$size的参数丢失或不解析为数组，则$size错误。

Example

考虑包含以下文档的inventory集合：

{ "_id" : 1, "item" : "ABC1", "description" : "product 1", colors: [ "blue", "black", "red" ] }
{ "_id" : 2, "item" : "ABC2", "description" : "product 2", colors: [ "purple" ] }
{ "_id" : 3, "item" : "XYZ1", "description" : "product 3", colors: [ ] }
{ "_id" : 4, "item" : "ZZZ1", "description" : "product 4 - missing colors" }
{ "_id" : 5, "item" : "ZZZ2", "description" : "product 5 - colors is string", colors: "blue,red" }

以下聚合管道操作使用$size运算符返回colors数组中的元素数：

db.inventory.aggregate([
   {
      $project: {
         item: 1,
         numberOfColors: { $cond: { if: { $isArray: "$colors" }, then: { $size: "$colors" }, else: "NA"} }
      }
   }
] )

该操作返回以下内容：

{ "_id" : 1, "item" : "ABC1", "numberOfColors" : 3 }
{ "_id" : 2, "item" : "ABC2", "numberOfColors" : 1 }
{ "_id" : 3, "item" : "XYZ1", "numberOfColors" : 0 }
{ "_id" : 4, "item" : "ZZZ1", "numberOfColors" : "NA" }
{ "_id" : 5, "item" : "ZZZ2", "numberOfColors" : "NA" }
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