$isArray (aggregation)

在本页面

• Definition

• Behavior

• Example

Definition

• $isArray
  • 3.2 版中的新功能。

确定操作数是否为数组。返回一个布尔值。

$isArray具有以下语法：

{ $isArray: [ <expression> ] }

Behavior

<expression>可以是任何有效的expression。有关表达式的更多信息，请参见Expressions。

Example

名为warehouses的集合包含以下文档：

{ "_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: [ ] }

下面的示例在将两者连接在一起之前检查instock和ordered字段是否为数组：

db.warehouses.aggregate([
   { $project:
      { items:
          { $cond:
            {
              if: { $and: [ { $isArray: "$instock" }, { $isArray: "$ordered" } ] },
              then: { $concatArrays: [ "$instock", "$ordered" ] },
              else: "One or more fields is not an array."
            }
          }
      }
   }
])

{ "_id" : 1, "items" : [ "chocolate", "butter", "apples" ] }
{ "_id" : 2, "items" : "One or more fields is not an array." }
{ "_id" : 3, "items" : [ "pears", "pecans", "cherries" ] }
{ "_id" : 4, "items" : [ "ice cream" ] }