nithyanand11

Try with objectId.value() option
Ex:
{
 'collectionName' : 'user',
  mapReduce : {
    map : 'function () {
                         emit (this._id,{ id : this._id.valueOf(), lastname:this["last-name"]});
                        }',
    reduce : 'function (key, values) {
                  var allDetails = [];
                  var user = { id: null, lastname : null};
                  for( var index = 0; index < values.length; index++)
                  {
                    user.lastname = values[index].lastname;
                    user.id = values[index].id;
                    allDetails.push(user);
                    user = { lastname : null};
                  }
                   return {list : allDetails}
               }',
    out : 'list'
  }
}