MongoDB ObjectIds

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Object IDs

You may have noticed that the database creates an unique object for each document:

db.deptCollection.find().pretty()

Examine the output carefully and you will notice for each department in the collection something like:

"_id" : ObjectId("5808e3d2ec0ff55100af2649")

An ObjectId is like a primary key found in relational databases, except in this case it is globally unique across the whole database, not just one table. It is similar to objectIds found in object-oriented programming languages.

ObjectIds created by MongoDB consist of 12 bytes:

ObjectID layout

Caption

0	1	2	3	4	5	6	7	8	9	10	11
time				machine			pid		inc

Where:

• time: system time
• machine: machine id
• pid: process id
• inc: incremental number