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 are a 12-byte BSON type, with the following structure:

ObjectID layout

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

Where:

• time: timestamp - the time in seconds
• machine: machine identifier
• pid: process identifier
• inc: auto-incrementing counter

Non-Default ObjectIds

You can assign an objectId yourself to a document, but you do need to ensure it will be globally unique, so treating it as a primary key is not a good idea.

db.deptCollection.insert(

{
   deptno: 100, 
   dname: 'ACCOUNTING',
   loc: 'NEW YORK',
   employees: [
    {
       _id: new ObjectId(),
       empno: 1111,
       ename: 'SMYTH',	
       job: 'MANAGER',
       mgr: 7839, 	
       hiredate: new Date('2012-08-19'),
       sal: 2450
    },
     {
       _id: new ObjectId(),

empno: 2222,

       ename: 'WILSON',
       job: 'TECHNICAN',
       mgr: 7782,
       hiredate: new Date(),
       sal: 1300
     }
   ]
  } 
)