Query using ObjectId in MongoDB

Question

I have a notes collection as:

{
    note: {
            type: String,
    },
    createdBy: {
        type: String,
        required: true
    },
}

where "createdBy" contains _id of a user from users collection. First Question: Should I define it as String or ObjectId?

Second Question: While querying the data as db.users.find({ createdBy: ObjectId(userid) },'notes'). Is it a O(1) operation? Or, do I have to create an index for that to be 0(1)?

Answer 1

1. If your user collection is using ObjectId then you would better also use ObjectId in notes collection since you may want to $lookup them.

2. Only _id field would create index in the begging of collection. You need to create index for createdBy since you want O(1) operation.