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}) in the documentation: http://www.mongodb.org/display/DOCS/Optimizing+Object+IDs#OptimizingObjectIDs-Sortbyidtosortbyinsertiontime

EDIT

Turns out the docs were wrong.

score 66 · Accepted Answer · edited Feb 19 '13 at 20:58

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I expect it's just a typo in the documentation. The _id field is primary key for every document. It's called _id and is also accessible via id. Attempting to use an id key may result in a illegal ObjectId format error.

That section is just indicating that the automatically generated ObjectIDs start with a timestamp so it's possible to sort your documents automatically. This is pretty cool since the _id is automatically indexed in every collection. See http://www.mongodb.org/display/DOCS/Object+IDs for more information. Specifically under "BSON ObjectID Specification".

A BSON ObjectID is a 12-byte value consisting of a 4-byte timestamp (seconds since epoch), a 3-byte machine id, a 2-byte process id, and a 3-byte counter. Note that the timestamp and counter fields must be stored big endian unlike the rest of BSON.

edited Feb 19 '13 at 20:58

dayyan

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answered Mar 14 '12 at 01:14

Leonard Garvey

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is the `_id` mandatory for each document of a collection? in other words, can I create a new document without the `_id` field? – Kevin Meredith Sep 20 '13 at 18:57
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@KevinMeredith As specified [here](https://docs.mongodb.com/manual/reference/bson-types/#objectid), yes, an `_id` field is mandatory. «In MongoDB, each document stored in a collection requires a unique `_id` field that acts as a primary key. If an inserted document omits the `_id` field, the MongoDB driver automatically generates an `ObjectId` for the `_id` field». – BowPark Jun 01 '17 at 09:30

score 15 · Answer 2 · answered Mar 14 '12 at 01:15

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The _id field is the default field for Bson ObjectId's and it is,by default, indexed.

_id and id are not the same. You may also choose to add a field called id if you want, but it will not be index unless you add an index.

It is just a typo in the docs.

answered Mar 14 '12 at 01:15

Bryan Migliorisi

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score 8 · Answer 3 · answered Aug 29 '16 at 12:45

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id is an alias for _id in mongoid.id would return the _id of the document. https://github.com/mongodb/mongoid/blob/master/lib/mongoid/fields.rb#L47

if the _id field is not specified an ObjectedId is generated automatically.

answered Aug 29 '16 at 12:45

tessie

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score 5 · Answer 4 · answered Aug 29 '16 at 18:52

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)

Difference between "id" and "_id" fields in MongoDB

4 Answers4

The _id field

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