Question

I have a unique ID field of type int in my documents. I’m doing ensureIndex on this field, but my documents still contain _id. Can I get rid of it?

Answer 1

From the docs …

If a user tries to insert a document without providing an _id field,
the database will automatically generate an object id and store it
the _id field.

However, you can assign your own value to _id …

The _id value may be of any type, other than arrays, so long as it is
a unique.

The better question is why would you create your own unique ID, build an index on it and remove the very useful ObjectID _id that is auto indexed?

All the official drivers use ObjectID and it has a lot of really good aspects to it:

• It has a timestamp built in (so you don’t need to store a date field
  in your document, and you can use it to order by date.)

• It is a “global, uniformly increasing sequence number” — i.e.
  it’ll still be unique across servers if you need to shard your data,
  etc.