provide $projection operator to get type of field

Syntax

{$type: "$fieldName"}

Examples

Input

{_id: NumberLong(4)}

Pipeline

db.coll.aggregate([{

    $project: {

        _id: {$type: "$_id"}

    }

}])

Output

{_id: "long"}

Additional Notes

• Type names will match the query $type operator.
• Note in a query, the $type will try to match elements inside of an array, so a {$type: 'array'} will only match nested arrays. This will not be the case in aggregation. The type of an array will be 'array'.
• Adding a $in expression (SERVER-6146) and a $switch expression (SERVER-10689) should make it easier to tell if something is numeric.

Original Description

If there was an equivalent of $type field (it can return bson type number or something else) it would be highly useful to do appropriate processing only if a field is a particular type.

For example, without knowing if a field is always an array, it's possible to get an error trying to $unwind or to get $size - if you could

{$project: {typeOfFoo:{$type:"$foo"}}}

you could now do different processing/operations depending on whether type is an array or not, or if it's a string vs number vs date, etc.