MongoDB聚合运算符:$size
文章目录
- MongoDB聚合运算符:$size
- 语法
- 使用
- 举例
$size聚合运算符返回数组中元素的数量。
语法
{ $size:
} 为可解析为数组的表达式。 使用
$size 的参数必须解析为数组。如果 $size 的参数丢失或不能析为数组,则 $size 错误。
举例
inventory集合有下列文档:
{ "_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" }
下面的聚合操作使用 $size 运算符返回color数组中的元素数量:
db.inventory.aggregate([ { $project: { item: 1, numberOfColors: { $cond: { if: { $isArray: "$colors" }, then: { $size: "$colors" }, else: "NA"} } } } ] )
操作返回下面的结果:
{ "_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" }