$type(聚合)
定义
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. Instead,
when passed an array as its argument, the $type
aggregation
operator returns the type of the argument, i.e. "array"
.
If the argument is a field that is missing in the input document,
$type
returns the string "missing"
.
The following table shows the $type
output for several
common types of expressions:
例子 | 结果 |
---|---|
|
|
|
|
|
|
|
|
|
|
|
|
注意
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
). Wrapping the array
[ 1, 2, 3 ]
in a $literal
expression
achieves the same result.
See operator expression syntax forms for more information.
可用类型
类型 | 数值 | 别名 | 注意 |
---|---|---|---|
double | 1 | "double" | |
字符串 | 2 | "string" | |
对象 | 3 | "object" | |
阵列 | 4 | "array" | |
二进制数据 | 5 | "binData" | |
未定义 | 6 | "undefined" | 已弃用。 |
ObjectId | 7 | "objectId" | |
布尔 | 8 | "bool" | |
Date | 9 | "date" | |
null | 10 | "null" | |
正则表达式 | 11 | "regex" | |
数据库指针 | 12 | "dbPointer" | 已弃用。 |
JavaScript | 13 | "javascript" | |
符号 | 14 | "symbol" | 已弃用。 |
32 位整数 | 16 | "int" | |
时间戳 | 17 | "timestamp" | |
64 位整型 | 18 | "long" | |
Decimal128 | 19 | "decimal" | |
Min key | -1 | "minKey" | |
Max key | 127 | "maxKey" |
If the argument is a field that is missing in the input document,
$type
returns the string "missing"
.
例子
This example uses a 集合 named coll
with
the following 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 : NumberLong(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.
db.coll.aggregate([{ $project: { a : { $type: "$a" } } }])
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" }