How to compare two fields value while querying in mongodb

To compare 2 fields of a document while querying in mongodb, we need to use $where.

Lets see the example to compare 2 fields of a document:

 Employee Collection  
           {    
             "_id" : 1,    
             "name" : {    
                "first" : "Abhi",    
                "last" : "Dev"    
             },    
             "department" : "finance",    
             "joineddate" : "2010-04-10"     
            },    
            {    
             "_id" : 2,    
             "name" : {    
                "first" : "Agrawal",    
                "last" : "Agrawal"    
             },     
             "joineddate" : "2006-07-07"     
            },    
            {    
             "_id" : 3,    
             "name" : {    
                "first" : "Dhruv",    
                "last" : "Agrawal"    
             },    
             "department" : "finance",    
             "joineddate" : "2010-09-07"     
            }  
           }   

Now lets say I need to fetch data which has first and last values are equal.

The query would be:

 db.employee.find({ $where : "this.name.last == this.name.first" })  

The output would be:

{    
             "_id" : 2,    
             "name" : {    
                "first" : "Agrawal",    
                "last" : "Agrawal"    
             }

How to update specific records of array field in collection using MongoDB

To update specific records of an array field in MongoDB, we can achieve that by using JavaScript with Mongo query. Below are the two ways to update the arrays. Lets see both ways to update the data.

Employee Collection:

 {  
           "_id" : 1,    
        "name" : {    
           "first" : "Abhi",    
           "last" : "Dev"    
        },    
        "profile":[ {"department" : "finance", "joineddate" : "2010-04-10"},  
                          {"department" : "marketing", "joineddate" : "2008-05-05"},  
                          {"department" : "hr", "joineddate" : "2006-08-05"}  
                          ]  
      },    
      {    
        "_id" : 2,    
        "name" : {    
           "first" : "Dhruv",    
           "last" : "Sharma"    
        },    
        "profile":[ {"department" : "retail", "joineddate" : "2013-07-10"},  
                          {"department" : "finance", "joineddate" : "2010-05-25"},  
                          {"department" : "hr", "joineddate" : "2007-08-05"}  
                          ]    
      }  

Objective is to update above array's joineddate field where department is finance.
Option 1: Updating the array using JavaScript:

 db.employee.find({}).  
      forEach(function(doc){  
           doc.profile.forEach(function(d){  
                if(d.department == "finance")  
                {       
                     d.joineddate = "2016-12-12"  
                }  
           });            
           db.employee.save(doc);  
      })   

Option 2: Updating array using Mongo Query operators update and set:

 var query = {  
   profile: {  
     $elemMatch: {  
       department: "finance",  
       joineddate: {$ne: "2016-12-12"}  
     }  
   }  
 };  
 while (db.employee.find(query).count() > 0) {  
   db.employee.update(  
     query,  
     { $set: { "profile.$.joineddate": "2016-12-12" } },  
     { multi: true }  
   );  
 }