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

Sergio is right, you have to do it outside of mongo.

You can do something like this:

var count = 0;
for(k in obj) {count++;}
print(count);

Answer 2

If You are work with Documents like the mongoose.Document, You can try to count all keys inside the doc after transform in a object, 'cause, before it, the doc just have commom properties:

Object.keys(document); // returns [ '$__', 'isNew', 'errors', '_doc', '$locals' ]
Object.keys(document).length; // returns ALWAYS 5

So, try cast to an object, after pass it to Object.keys

console.dir(
    Object.keys(document.toObject()) // Returns ['prop1', 'prop2', 'prop3']
);

And to show the keys count

console.log(
    Object.keys(document.toObject().length)
); // Returns 3

And Be Happy!!!

Answer 3

const MongoClient = new require("mongodb").MongoClient(
  "mongodb://localhost:27017",
  {
    useNewUrlParser: true,
    useUnifiedTopology: true
  }
);
(async () => {
  const connection = await MongoClient.connect();
  const dbStuff = await connection
    .db("db")
    .collection("weedmaps.com/brands")
    .find({})
    .toArray();  // <- Chuck all it into an Array
  for (let thing of dbStuff) {
    console.log(Object.keys(thing).length);
  }
  return await connection.close();
})();

Answer 4

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

Answer 5

If you are in Mongo console, you can write javascript to do that.

doc = db.coll.findOne({}); nKeys =0; for( k in doc){nKeys ++;} print(nKeys);

This will still be considered "outside the mongo" though.

Answer 6

In Mongo (as in most NoSQL solutions) it's usually a good idea to pre-calculate such values if you want to do queries on them later (e.g. "where the number of different keys > 12") so maybe you should consider adding a new field "keyCount" which you increment every time a new key is added.