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

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.

Answer 3

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.

Answer 4

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 5

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 6

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 7

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!!!