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  1. Core Server
  2. SERVER-13447

provide $projection operator to get type of field

    Details

      Description

      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.

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                • Created:
                  Updated:
                  Resolved: