I have 2 separate groups of users on my website. One is user and the other one is admin. Currently I put them in 2 different collections user and admin and login respectively. My questions is:

  1. Mongodb generates _id automatically for both user and the admin collection. Since they're in 2 different collections chances are one day you created one user and one admin with the same _id right? So it seems the only way is to check all documents in both dbs and make sure your newly created user (or admin) does not have repeated _id right? Is there something I can do natively in mongodb to make this happen without checking?

  2. I can also put all users and admins in one collection. This way I don't have a problem but I am not sure if I should do this.

p.s: because I have a table keeping track of user / admin balances so I certainly don't want the _id to repeat p.s2: the db is taking longer and longer to query. Also my ORM creates user instances and generated _id automatically for me. I am not sure if I should just generate an _id and create it this way...

Any comments are greatly appreciated. Thanks!!

2 Answers 2


You should not have two collections, that will give you headache.

Have a user collection which has admin and regular users. You can then query users as a whole or look for a given type only. So if you are looking for user id x you don't have two search two collections.

As for the question on duplicate ids with 2 collections, it simply depends how the ids are generated. You say your ORM does it... so you will need to check the doc on how it does it.

  • got it. about 1 it is that I am dealing with transactions and I don't want things like collide id to happen. Although I know it probably wouldn't. Also I thought when I separate the collection I can do map-reduce more efficiently on some scenarios but maybe I really shouldn't worry about this kind of things. But I agree what you said is better!! Thanks! Jan 2, 2015 at 2:27
  1. About _id generation, as long as you don't specify _id explicitly, Mongo would handle the id for you. It generates different nonce for different collection. So you should not need to worry about duplicated _id across different collection. Reference here.

  2. I agree with other answer, use same collection and put an extra field to differentiate regular user and admin

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