[{"_id":"mongo_id","name":"test user","email":"test_4@gmail.com","full_name":"","first_name":"","last_name":"","mobile_number":"","email_id":"test_4@gmail.com","profile_photo":"","roles":[],"favorites":[{obj1:1},{obj2:2},{obj3:3},{obj4:4}]}]

db.collection.update({_id:"mongo_id","favorites.50":{"$exists":false}},{"$push":{favorites:{obj:10}}})