ObjectId ( _id ) as Primary Key: MongoDB

Advertisement:

In this video tutorial we shall learn more about ObjectId( _id ) of MongoDB document.

ObjectId is a primary key in any mongoDB document.
We can’t change document id once it has been created and inserted.
If we do not explicitly assign value to _id, then it’ll be automatically assigned and inserted.
We can’t have same document id for more than 1 document.

These ObjectId’s are of BSON datatype.
It’s a 12-byte value.

Basically, each _id is a combination of ..
– Time the document/record was saved/inserted
– Host name of the machine/server it’s running on
– Process id of the server process
– and a random incremental number

Example documents

mongoDB-collection-document

Since the ObjectId‘s are formed taking time of insertion of document into consideration, we can extract this time using getTimestamp() method.

1
db.info.find()[0]._id.getTimestamp()

Custom _id values
We can explicitly define the _id value and in such case system won’t add it for us.

1
2
3
4
5
db.info.insert({
   _id    : 2,
   name   : "Google",
   product: "Google Glass"
})

output

1
2
3
4
5
6
>db.info.find().forEach(printjson)
{
	"_id"    : 2,
	"name"   : "Google",
	"product": "Google Glass"
}

If you insert ObjectId yourselves, then having a separate ‘document_created_at’ field becomes necessary depending on your application requirements.

For that, you can use a inbuilt method Date()

1
2
> new Date()
ISODate("2013-05-01T07:55:12.467Z")

Date is actually a class, but it calls upon the constructor method, hence returning current system/server timestamp, in ISO format.

Remove / Delete / Drop Document

1
db.info.remove({ _id: 2 })

To remove a document, simply pass in a unique { key: value } pair and the document gets deleted/removed.

Primary Key in MongoDB: ObjectId


You need to a flashplayer enabled browser to view this YouTube video

YouTube Link: http://www.youtube.com/watch?v=YCZV5N01Oqs [Watch the Video In Full Screen.]



Navigation of Documents
To navigate between documents, use the index numbers, and to select particular field use it’s key name.

To get the _id of first document

1
db.info.find()[0]._id

0 is the index of first document or record. _id is the field/key name.

1 thought on “ObjectId ( _id ) as Primary Key: MongoDB”

Leave a Reply

Your email address will not be published. Required fields are marked *