MongoDB ObjectIds

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Object IDs

Object IDs were introduced in section 2 and you may have noticed that the database creates an unique object for each document if you have not created one yourself:

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 unique within the collection. Alternatively, you can use the ObjectId() function to add one, which again will be unique within the collection.

The following examples creates a new projectCollection, with two projects:

db.projCollection.insert( 
{ _id: 10,
  projno: 110, 
  proj_name: 'Oracle Project',
  budget: 10000
  }
 )
 
db.projCollection.insert(
{ _id: 20,
  projno: 120, 
  proj_name: 'MongoDB Project',
  budget: 20000
  }  
)

If you try and reuse the same objectId:

db.projCollection.insert( 
{ _id: 10,
  projno: 210, 
  proj_name: 'Other Oracle Project',
  budget: 10000
  }
 )

You will get a E11000 duplicate key error collection error message.

This is because user generated _ids must be unique within the collection. If you want to reference the _id outside the collection, you need to use a system generated id:

 db.projCollection.insert(  {
  projno: 140, 
  proj_name: 'ObjectId Project',
  budget: 25000
 } )

By not defining the _id, the system will generate one automatically.

Check what _id the project has:

db.projCollection.find({projno: 140}).pretty()

Then add a new department using the object id shown for a project field:

db.deptCollection.insert( {
  deptno: 70,
  dname: 'OBJECT Test',
  loc: 'STOCKPORT',
  employees: [ {
     empno: 81999,
     ename: 'Perry',
     project:  ObjectId("your_project_id")
     } ]
  }
)

Where you need to replace your_project_id with the _id of the project above, for example: "580a8701105d532281cdd3aa"

Using Variables

The above method is not very user-friendly and prone to errors. Alternatively, you can save the object ID into a variable and use that as a value.

First find the object ID:

 var oid = db.projCollection.findOne({projno: 140})

findOne() must be used instead of find(), since the latter returns a cursor, even if there is only one document that matches the query condition.

Then when adding a new document this variable can be used for the object id:

db.deptCollection.insert( {
 deptno: 80,
 dname: 'OID Test',
 loc: 'WOLVERHAMPTON',
 employees: [ {
    empno: 9999,
    ename: 'Sanjit',
    project: oid 
    } ]
  }
)

Querying with ObjectIds

To see our new department:

db.deptCollection.find({deptno: 70}).pretty()

The ObjectId is a manual document reference. A second query is needed to return the project's results:

db.projCollection.findOne( { _id: ObjectId("your_object_id") })

Next step

Return to the Workbook.