[{"user":{"id":1,"name":"A","email":"A@a"}},{"user":{"name":"B","email":"B@b"}},{"user":{"email":"C@c"}},{"user":{}}]

db.collection.aggregate([{$project:{userDetail:{$ifNull:["$user.id",{$ifNull:["$user.name",{$ifNull:["$user.email",""]}]}]}}}])