In meteor/mongo collections, is the _id field unique in its collection or in the entire database?

Question

I want to check that if I use _id fields that refer to documents from different collections, I will never have a duplicate _id, i.e. used in 2 different collections inside the same database.

Using meteor (so both in minimongo and mongodb), is the _id field unique in its collection or in the entire database?

What way of generating `_id` values do you use? Are they auto-generated by Meteor? Please provide an example of an `_id` value. — Dmytro Shevchenko, Dec 09 '15 at 13:31
Here is an example: `qhrEnh36tthTswiYv`. I assume this is generated by mongo. I use the `_id` values attached to every mongo document I produce using meteor. — Billybobbonnet, Dec 09 '15 at 13:34
Ok, I guess this question answers mine, even if it is not really the same: http://stackoverflow.com/a/33332339/3793161 Bottom line: they are generated by meteor and they are unique among the whole database. Correct me if I'm wrong, and if you think I should flag my question as duplicate — Billybobbonnet, Dec 09 '15 at 13:39
I don't think you should mark your question as duplicate. This is a different question, and hopefully I have provided a satisfactory answer to it. — Dmytro Shevchenko, Dec 09 '15 at 13:41

score 3 · Accepted Answer · answered Dec 09 '15 at 13:38

3

The _id values you have in your database are generated by Meteor using Random.id(). These are unique across all collections.

Please note that the uniqueness of _id values in MonogoDB is ensured on the collection level, meaning that there is always a unique index on the _id field for every collection. There is no MongoDB mechanism in place that would ensure _id uniqueness across collections.

In any case, it is quite a safe assumption that Meteor's random IDs will never collide.

answered Dec 09 '15 at 13:38

Dmytro Shevchenko

33,431
6
51
67

1

As you said, this is a "safe assumption". But, still, that's an assumption. Are they guaranteed to be unique, or not (suppose I have millions and millions of documents and resources are not an issue)? I mean, is there a mechanism in place where if the Random.id() was already generated before that it will be skipped and a new one generated? Because, if not, then there *can* be collisions (even if they are unlikely, but, people have been known to win the lottery). – trusktr Feb 20 '16 at 21:21
How can we make sure that the id Meteor generates would not collide with existing ones on a collection? (Throwing an error is not ok cause it would break the operation and prevent the data from being saved) – cenk Feb 25 '17 at 00:42

In meteor/mongo collections, is the _id field unique in its collection or in the entire database?

1 Answers1