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- $type (aggregation)
$type (aggregation)¶
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Definition¶
- $type¶
3.4 新版功能.
Returns a string that specifies the BSON type of the argument.
$type has the following operator expression syntax:
{ $type: <expression> }
The argument can be any valid expression.
Behavior¶
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".
| Example | Results |
|---|---|
| { $type: "a" } | "string" |
| { $type: /a/ } | "regex" |
| { $type: 1 } | "double" |
| { $type: NumberLong(627) } | "long" |
| { $type: { x: 1 } } | "object" |
| { $type: [ [ 1, 2, 3 ] ] } | "array" |
注解
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.
Example¶
This example uses a collection named coll with the following documents:
{ _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" }
}
}])
The operation returns the following:
{ _id: 0, "a" : "double" }
{ _id: 1, "a" : "array" }
{ _id: 2, "a" : "object" }
{ _id: 3, "a" : "string" }
{ _id: 4, "a" : "long" }
{ _id: 5, "a" : "missing" }