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MongoDB stores all data records in separate BSON documents. An identifier is provided to uniquely identify each document.

The identifier helps users uniquely locate the document when searching for other documents.

With the help of identifiers, queries can be easily performed and results can be generated accordingly.

In each collection, each document has a unique _idfield as its primary key. _idThe default format is the document's ObjectID.

ObjectID Features

An ObjectID acts as an identifier for a single document within a particular collection. An ObjectID is a 12-byte BSON type field.

The fields are divided as follows:

1. The Unix timestamp of a single document is represented by the first four bytes.
2. The machine ID of the current MongoDB server is represented by the following three bytes.
3. The process ID is represented by the next two bytes.
4. For incrementing the ObjectID, the last three bytes are reserved.

ObjectID format

You can define an ObjectID when you create a document. There are two ways to assign an ObjectID to a document.

1. Upon creation, the user assigns an ObjectID to the document itself.
2. The document uses the default ObjectID assigned to it by the MongoDB server.

ObjectIDs are defined in the following format:

ObjectId(<hexadecimal>)

The parameter used to define the ObjectID <hexadecimal>is optional. If the user _iddefines a value in the field, that value will be assigned as the ObjectID to the document.

Otherwise, the MongoDB server assigns an ID to the document.

Difference between ObjectID and $oid in MongoDB

There is no difference between ObjectID and $oid in MongoDB server. They are just different serialization formats introduced by the new shell.

Object IDs are represented with the help of $oid in the MongoDB server. The server uses Strict MongoDB Extended JSON; therefore, object IDs are represented as follows:

{ "$oid": "<id>" }

As shown above, you can use the $oid term when searching for a specific document. You can use $oid when writing queries.

On the other hand, ObjectID can be used to create documents. In addition, ObjectID methods can be used to get specific values ​​from the 12-byte Object ID.

In conclusion, there is not much difference between these two entities in MongoDB server.

Insert a document into a collection using ObjectID

As mentioned in the previous section, the ObjectID field takes an optional hexadecimal parameter. If you do not _idset a value for the field when you create a document, the MongoDB server assigns a default unique value to the documents in the collection.

The syntax for inserting a single document into a collection is as follows:

db.collectionName.insertOne()

This code snippet inserts a document into the collection with _idthe default value in the field set by the server.

The syntax for inserting multiple documents into a collection is as follows:

db.collectionName.insertMany()

This code snippet inserts multiple documents into a collection and _iduses default values ​​in the fields of the server settings.

Set the ObjectID field

The user can set the field when creating a document _idbecause it is unique. The syntax for setting ObjectID or $oid is as follows:

db.collectionname.insertOne({"_id":"1789473"})

Here is an example of setting the ObjectID of a document in a collection. Assume the following entity:

1. Database name: officedb
2. Set name: employee_officedb

Use the following command to create a document in the MongoDB server:

> use officedb
switched to db officedb
> db.createCollection("employee_officedb")
{ "ok" : 1 }
> db.employee_officedb.insert({ name : "asad", rank : 23})
WriteResult ({ "nInserted" : 1})
> db.employee_officedb.find().pretty()
{
        "_id" : ObjectId ("8e12bn2a0ty562888ab93711"),
        "name" : "asad"
        "rank"  : 23
}

ObjectID methods

Four methods extend the concept of ObjectID in MongoDB. Each method has different functionality, as described below.

1. str Method: This is used to get the object id in hexadecimal string format.
2. The ObjectId.getTimestamp() method returns the timestamp portion of an object as a date.
3. ObjectId.valueOf() method: This method returns the hexadecimal format of the given string literal.
4. ObjectId.toString() method: This method returns the object ID in a string format represented by JavaScript.

Summarize

There is a lot of room to explore ObjectID and $oid in MongoDB server. There is not much difference between the two except the representation.

Some shells use ObjectID, while others use $oid. Therefore, data manipulation and storage depends on the version of the shell installed on your device.

For the definition of documents and their IDs, use ObjectID. On the other hand, when writing and running queries, use $oid to search the entire collection to find the required document.