Are MongoDB document IDs unique to the entire database or just that collection?

Question

I'm using MLab to host a MongoDB database for a node.js project I'm working on. I want to use the uids automatically given to document to identify specific documents across collections. I was wondering if the default ids are unique only in the collection the document is in, or if they are unique throughout the entire database.

{
    "_id": { // <<<< this is what I'm talking about
        "$oid": "5c0754d8d0d1c81b400690a3"
    },
    "owned": [],
    "equipped": [],
    "prospects": [],
    "username": "Urist",
    "password": "dcddb75469b4b4875094e14561e573d8",
    "date": {
        "$date": "2018-12-05T04:32:24.690Z"
    },
    "trades": [],
    "__v": 0
}

https://stackoverflow.com/questions/4677237/possibility-of-duplicate-mongo-objectids-being-generated-in-two-different-colle — numbers1311407, Dec 05 '18 at 21:19

score 1 · Accepted Answer · answered Dec 05 '18 at 22:01

_id must be unique across a single collection. It is not necessary for _id to be unique for documents in different collection or database.

In https://docs.mongodb.com/manual/core/document/#field-names:

The field name _id is reserved for use as a primary key; its value must be unique in the collection, is immutable, and may be of any type other than an array.

Are MongoDB document IDs unique to the entire database or just that collection?

1 Answers1