Possibility of duplicate Mongo ObjectId's being generated in two different collections?

Question

Is it possible for the same exact Mongo ObjectId to be generated for a document in two different collections? I realize that it\'s definitely very unlikely, but is it possi

Answer 1

ObjectIds are generated client-side in a manner similar to UUID but with some nicer properties for storage in a database such as roughly increasing order and encoding their creation time for free. The key thing for your use case is that they are designed to guarantee uniqueness to a high probability even if they are generated on different machines.

Now if you were referring to the _id field in general, we do not require uniqueness across collections so it is safe to reuse the old _id. As a concrete example, if you have two collections, colors and fruits, both could simultaneously have an object like {_id: 'orange'}.

In case you want to know more about how ObjectIds are created, here is the spec: http://www.mongodb.org/display/DOCS/Object+IDs#ObjectIDs-BSONObjectIDSpecification