MongoDB: how to count number of keys in a document?

Question

Lets say a document is :

{  a: 1, b: 1, c: 2,  .... z: 2 } 

How can I count the number of keys in such document?

Thank you

Answer 1

Quite possible if using MongoDB 3.6 and newer though the aggregation framework. Use the $objectToArray operator within an aggregation pipeline to convert the document to an array. The return array contains an element for each field/value pair in the original document. Each element in the return array is a document that contains two fields k and v.

The reference to root the document is made possible through the $$ROOT system variable which references the top-level document currently being processed in the aggregation pipeline stage.

On getting the array, you can then leverage the use of $addFields pipeline step to create a field that holds the counts and the actual count is derived with the use of the $size operator.

All this can be done in a single pipeline by nesting the expressions as follows:

db.collection.aggregate([     { "$addFields": {         "count": {             "$size": {                  "$objectToArray": "$$ROOT"             }         }     } }      ]) 

Example Output

{     "_id" : ObjectId("5a7cd94520a31e44e0e7e282"),     "a" : 1.0,     "b" : 1.0,     "c" : 2.0,     "z" : 2.0,     "count" : 5 } 

---

To exclude the _id field, you can use the $filter operator as:

db.collection.aggregate([     {         "$addFields": {             "count": {                 "$size": {                      "$filter": {                         "input": { "$objectToArray": "$$ROOT" },                         "as": "el",                         "cond": { "$ne": [ "$$el.k", "_id" ] }                     }                 }             }         }     }      ]) 

or as suggested by 0zkr PM simply add a $project pipeline step at the beginning:

db.collection.aggregate([     { "$project": { "_id": 0 } },     { "$addFields": {         "count": {             "$size": {                  "$objectToArray": "$$ROOT"             }         }     } }      ]) 

Answer 2

There's no built-in command for that. Fetch this document and count the keys yourself.