I’ve just run db.system.indexes.find({"unique": true}); against mongo shell and it didnt show any index on _id field of any collection 
If they are not - how can I ensure their uniqueness? Eg when I want to have my own _id generation instead of Random's package. Does ensureIndex works with _id field or something?..
_id should be unique. Mongo controls that not Meteor
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thx, good to know 
I thought in Meteor, Meteor controls the _id field? It’s generated via Meteor’s Random.id(), whereas Mongo’s _id is a full object that contains a unique ID as well as a timestamp, and maybe other metadata.
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Yes, but Random.id() is still pretty unique. One in 10^30 something…
So basically if every person on earth generated one id per second for a lifetime, the chance of any two of them to be the same is still one in a hundred billion…
It is funny you would say that, because we are hitting non-unique ids on a project today. Where did you find that 10^30 number?
I don’t believe that the Mongo API would allow Meteor to insert duplicate _id’s which is a unique identifier, but someone from MDG or maybe Sacha would likely be better to answer.
Wow, really? What happened, and how did Meteor resolve it? You have the option of making Meteor use Mongo style IDs instead, if it’s an issue.
Random.id() is a 17 digit base 60 number. 60^17 = 1.7e30.
This is of course assuming true randomness, something which is hard, but something I was hoping someone competent who developed Random would have made at least a decent attempt at, given that the science is well known.
If you are hitting duplicate ids using anything short of an insane number of objects, I would say Random is seriously broken.
Yeah, really interesting to know!
- does Random generates not-random strings?
- does mongo allow non-unique _id’s in collecitons??
Sorry to bump this old thread up but we are currently developing app with Meteor that will collect huge amount of data in a collection (millions) and would like to know if these _id fields generated by Meteor are really unique or should we preferably move to using mongo’s ObjectID format for production? If _id generated by mongo is not checking if there is another document with that _id in the db, what happens when trying to insert something to db and the _id generated by Meteor is not unique, adds duplicate _id or rejects adding it?
Would be great to get answer for this from MDG 
They are random (pseudo-random). On the server, they will be generated using server entropy, but random (or pseudo-random) numbers cannot guarantee uniqeness (for a simplified argument for this, look at the birthday paradox). For guaranteed uniqueness you really need something like an auto-increment.
MongoDB’s internal objectid includes a bunch of stuff including timestamp information, and in most use cases will be better at collision avoidance than a random number. However, Meteor’s implementation of this is still based on a random number.
Rejects. _id has a unique constraint.