Mongodb query document array field length without fetch the array

Question

Suppose I have a collection with an array field:

{
  title: String,
  users: [String],
}

Now I need to query a list of documents from the collection, using only the length of users in each of it, without actual elements in users array (because it maybe large, it won't be worth to fetch them from db and count the length):

Collection.find({}).sort({ _id: -1 })

Is it possible?

Do you want to count the elements inside the users array?

Ashh
– Ashh

2018-12-13 09:28:35 +00:00
Commented Dec 13, 2018 at 9:28 — Ashh
– Ashh, Commented Dec 13, 2018 at 9:28
@AnthonyWinzlet yeah, exactly

wong2
– wong2

2018-12-13 09:32:35 +00:00
Commented Dec 13, 2018 at 9:32 — wong2
– wong2, Commented Dec 13, 2018 at 9:32

Ashh · Accepted Answer · 2018-12-13 09:55:44Z

2

You can use $size aggregation operator to find the length of the users array

db.collection.aggregate([
  { "$project": { "userLength": { "$size" :"$users" }}}
])

If you want to find all the users array length exists in the database

db.collection.aggregate([
  { "$group": {
    "_id": null,
    "allUsersArrayLength": { "$sum": { "$size": "$users" }}
  }}
])

edited Dec 13, 2018 at 9:55

answered Dec 13, 2018 at 9:34

Ashh

46.6k16 gold badges111 silver badges137 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ashh Over a year ago

It will not affect the performance of the query. Only add a new key with every document. Better then "finding all the document" and check the length by iterating. And even most of the things a database can better do with respect to javascript code.

Collectives™ on Stack Overflow

Mongodb query document array field length without fetch the array

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related