Difference between “id” and “_id” fields in MongoDB

Question

Is there any difference between using the field ID or _ID from a MongoDB document?

I am asking this, because I usually use \"_id\", however I saw this sort({id:-1})

Answer 1

My two cents:

The _id field

MongoDB assigns an _id field to each document and assigns primary index on it. There're ways by which we can apply secondary indices as well. By default, MongoDB creates values for the _id field of type ObjectID. This value is defined in BSON spec and it's structured this way:

ObjectID (12 bytes HEX string) = Date (4 bytes, a timestamp value representing number of seconds since the Unix epoch) + MAC address (3 bytes) + PID (2 bytes) + Counter (3 bytes)