[{"_id":"1","Gender":"male","car":"bmw","house":"2bhk","married_to":"kalpu"},{"_id":"2","Gender":"female","car":null,"house":"3bhk","married_to":"kalpu"},{"_id":"3","Gender":"female","car":"audi","house":"1bhk","married_to":"deepa"},]

db.collection.aggregate([{$group:{_id:null,Gender:{"$addToSet":"$Gender"},car:{"$addToSet":"$car"},house:{"$addToSet":"$house"},married_to:{"$addToSet":"$married_to"},}}])