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"    
             }