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

Adding 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. Instead you can use the ObjectId() function to add one.

Since trying to update individual employee details has caused problems, adding an objectId should help this:

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
     }
   ]
  } 
)

Check what difference that has made to this new department:

db.deptCollection.find({"deptno":100}).pretty()

Querying with ObjectIds

db.deptCollection.find({ "employees._id" : ObjectId("580a0e5bf090aeb82af5566c")}).pretty()

Still returns whole array!

Updates with ObjectIds

The objectId can be used to update a record. For example, we want to update employee 2222.