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# MongoDB ObjectId * * * In the previous few chapters, we have already used MongoDB's ObjectId. In this chapter, we will understand the structure of ObjectId. ObjectId is a 12-byte BSON type data, with the following format: * The first 4 bytes represent a timestamp * The next 3 bytes are the machine identifier code * The following two bytes consist of the process ID (PID) * The last three bytes are a random value. Documents stored in MongoDB must have an "_id" key. The value of this key can be of any type, and by default, it is an ObjectId object. In a collection, each document has a unique "_id" value to ensure that every document in the collection can be uniquely identified. The main reason MongoDB uses ObjectId instead of other more conventional methods (such as auto-incrementing primary keys) is that synchronizing auto-incrementing primary key values across multiple servers is both effortful and time-consuming. * * * ## Creating a New ObjectId Use the following code to generate a new ObjectId: >newObjectId = ObjectId() The above statement returns the following uniquely generated id: ObjectId("5349b4ddd2781d08c09890f3") You can also use the generated id to replace MongoDB's automatically generated ObjectId: >myObjectId = ObjectId("5349b4ddd2781d08c09890f4") * * * ## Creating a Timestamp for a Document Since ObjectId stores a 4-byte timestamp, you do not need to save a timestamp field for your document. You can use the getTimestamp function to get the creation time of the document: >ObjectId("5349b4ddd2781d08c09890f4").getTimestamp() The above code will return the document creation time in ISO format: ISODate("2014-04-12T21:49:17Z") * * * ## Converting ObjectId to String In some cases, you may need to convert an ObjectId to a string format. You can use the following code: >new ObjectId().str The above code will return a string in GUID format: 5349b4ddd2781d08c09890f3