Database Manual / Reference / Query Language / Expressions

$type (aggregation)（聚合）

Definition定义

$type

Returns a string that specifies the BSON type of the argument.返回一个字符串，指定参数的BSON类型。

$type has the following operator expression syntax:具有以下运算符表达式语法：

{ $type: <expression> }

The argument can be any valid expression.参数可以是任何有效的表达式。

Tip

  • $isNumber - checks if the argument is a number.检查参数是否为数字。
  • $type (Query)（查询） - filters fields based on BSON type.根据BSON类型筛选字段。

Behavior行为

$type

Unlike the $type query operator, which matches array elements based on their BSON type, the $type aggregation operator does not examine array elements. 与根据BSON类型匹配数组元素的$type查询运算符不同，$type聚合运算符不检查数组元素。Instead, when passed an array as its argument, the $type aggregation operator returns the type of the argument, i.e. "array".相反，当传递一个数组作为其参数时，$type聚合运算符返回参数的类型，即"array"。

If the argument is a field that is missing in the input document, $type returns the string "missing".如果参数是输入文档中缺少的字段，$type将返回字符串"missing"。

The following table shows the $type output for several common types of expressions:下表显示了几种常见表达式类型的$type输出：

Example示例Results结果
{ $type: "a" }"string"
{ $type: /a/ }"regex"
{ $type: 1 }"double"
{ $type: Long(627) }"long"
{ $type: { x: 1 } }"object"
{ $type: [ [ 1, 2, 3 ] ] }"array"

Note

In the case of a literal array such as [ 1, 2, 3 ], enclose the expression in an outer set of array brackets to prevent MongoDB from parsing [ 1, 2, 3 ] as an argument list with three arguments (1, 2, 3). 对于像[1,2,3]这样的文字数组，将表达式括在一组外部数组括号中，以防止MongoDB将[1,2,3]解析为具有三个参数（1,2,3）的参数列表。Wrapping the array [ 1, 2, 3 ] in a $literal expression achieves the same result.将数组[1,2,3]包装在$literal表达式中可以获得相同的结果。

See operator expression syntax forms for more information.有关更多信息，请参阅运算符表达式语法形式。

Available Types可用类型

Type类型NumberAliasNotes备注
Double双精度浮点数1"double"
String字符串2"string"
Object3"object"
Array数组4"array"
Binary data5"binData"
Undefined6"undefined"Deprecated.
ObjectId7"objectId"
Boolean布尔值8"bool"
Date9"date"
Null10"null"
Regular Expression11"regex"
DBPointer12"dbPointer"Deprecated.
JavaScript13"javascript"
Symbol14"symbol"Deprecated.
JavaScript with scope15"javascriptWithScope"Deprecated.
32-bit integer16"int"
Timestamp17"timestamp"
64-bit integer18"long"
Decimal12819"decimal"
Min key-1"minKey"
Max key127"maxKey"

If the argument is a field that is missing in the input document, $type returns the string "missing".如果参数是输入文档中缺少的字段，$type将返回字符串"missing"。

Example示例

This example uses a collection named coll with the following documents:此示例使用名为coll的集合和以下文档：

{ _id: 0, a : 8 }
{ _id: 1, a : [ 41.63, 88.19 ] }
{ _id: 2, a : { a : "apple", b : "banana", c: "carrot" } }
{ _id: 3, a :  "caribou" }
{ _id: 4, a : Long(71) }
{ _id: 5 }

The following aggregation operation uses the $type operator to display the type of field a for all documents as part of the $project stage.以下聚合操作使用$type运算符显示所有文档的字段a的类型，作为$project阶段的一部分。

db.coll.aggregate([{
    $project: {
       a : { $type: "$a" }
    }
}])

The operation returns the following:该操作返回以下内容：

{ _id: 0, "a" : "double" }
{ _id: 1, "a" : "array" }
{ _id: 2, "a" : "object" }
{ _id: 3, "a" : "string" }
{ _id: 4, "a" : "long" }
{ _id: 5, "a" : "missing" }