$type in Mongodb aggregation operation

In the mongodb query statement, the user can query the document data that matches the field type by specifying the $type value. In mongodb aggregation operations, the $type operator also exists. This article introduces $type in aggregation operations.

definition

Returns a string representing the BSON type of the passed parameter. In aggregation operations, follow the following syntax to return type information

{$type: <expression>}

Among them, expression can be any legal expression such as field name, operation, etc.

Behavior

Different from $type usage in query filters. $type in aggregation operations is used to return the result type in the expression. If an array is passed in as a parameter, the array type is returned.

When a field name is specified as an argument, $type returns the string "missing" when the field does not exist in the document.

The following table describes the results obtained when different parameters are passed in.

expression	result
{$type: "a"}	"string"
{$type: /a/}	"regex"
{$type: 1}	"double"
{$type: NumberLong(627)}	"long"
{$type: {x: 1}}	"object"
{$type: [ [1, 2, 3] ]}	"array"

Among them, in order to prevent the array [1, 2, 3] from being parsed into a parameter list, the array is converted into a literal meaning.

application

There are the following 6 documents in the collection coll.

db.coll.insertMany([
    {_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},
    ])

Use aggregate query to query the type of field a

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

/* 1 */
{
	"_id" : 0,
	"a" : "int"
},

/* 2 */
{
	"_id" : 1,
	"a" : "array"
},

/* 3 */
{
	"_id" : 2,
	"a" : "object"
},

/* 4 */
{
	"_id" : 3,
	"a" : "string"
},

/* 5 */
{
	"_id" : 4,
	"a" : "long"
},

/* 6 */
{
	"_id" : 5,
	"a" : "missing"
}

Let’s try the type of array returned in the example above. When passing in the array directly, mongodb returned an error. Mongodb parses the directly passed array into 3 parameters. $type can only accept one parameter

//直接传入一个数组
db.coll.aggregate([{
    $project: {
        a: {$type: [1,2,3]}
    }
}])
{
	"message" : "Invalid $project :: caused by :: Expression $type takes exactly 1 arguments. 3 were passed in.",
	"ok" : 0,
	"code" : 16020,
	"codeName" : "Location16020"
}

Therefore we need to pass the literal meaning of the array as an expression to $type as a parameter.

db.coll.aggregate([{
    $project: {
        a: {$type: {$literal: [1,2,3]}} //使用$literal传递数组的字面意义，不要被解析
    }
}])

Or pass the entire array as a parameter to $type

db.coll.aggregate([{
    $project: {
        a: {$type: [[1,2,3]]}
    }
}])

Here, $type is followed by an array, and only one element in the array is an array of [1,2,3]. When mongodb parses, it gets this array as a parameter.