ObjectId ( _id ) as Primary Key: MongoDB

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

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

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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.

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

output

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>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()

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> 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

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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

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

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

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