How to form a exact query for finding id's inside array

Question

I've a document like this:

{
    "_id" : ObjectId("xxx"),
    "users" : [ 
        {
            "_id" : ObjectId("xxx")
        }, 
        {
            "_id" : ObjectId("yyy")
        }
    ]
}

users is an array, which can contain two or more than two objects.
If I've to request strictly two objects via a mongo query, how can I do it?

.find({ 
    "users._id" : { $in: [ObjectId("xxx"), ObjectId("yyy")] }
})

This above query will fetch all the docs that've either've xxx user or yyy user in them. How can I form a query where if the both xxx and yyy are only there, then only that doc is to be fetched.
Let me know if you need more info to understand the problem. And huge thanks in advance for the help.

try $all operator instead of $in.

turivishal
– turivishal

2021-05-27 09:00:10 +00:00
Commented May 27, 2021 at 9:00 — turivishal
– turivishal, Commented May 27, 2021 at 9:00
yes. $all is working fine. thanks!

Yash
– Yash

2021-05-27 10:01:18 +00:00
Commented May 27, 2021 at 10:01 — Yash
– Yash, Commented May 27, 2021 at 10:01

Tom Slabbaert · Accepted Answer · 2021-05-27 09:10:49Z

1

You should use $all to force the need for all ids in the array to match but it is not enough on it's own as you'll still match documents with those two users + other users.

To combat that you will also have to make sure the users array is of size 2, you can do it like so:

db.collection.find({
  $and: [
    {
      "users._id": {
        $all: [
          ObjectId("609a80e3c548140e5859d6de"),
          ObjectId("60136b272c1e946545da6204")
        ]
      }
    },
    {
      "users.2": {
        $exists: false
      }
    }
  ]
})

Mongo Playground

answered May 27, 2021 at 9:10

Tom Slabbaert

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2 Comments

Yash Over a year ago

Awesome. It's working even if we have more than two users. But can you please tell me what this "user.2" is doing. I understand that it is checking for length of 2, but can you point me out to mongo docs, so I can read more about it.

Tom Slabbaert Over a year ago

'users.2' is the 2 index in an array, so if users.2 exists than it means the array has at least 3 items in it.

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