[{subs:[{subsub:null},{subsub:{a:"a",b:"b"}},{subsub:{a:"c",b:"d"}}]},{subs:[{subsub:null},{subsub:{a:"e",b:"f"}},{subsub:{a:"g",b:"h"}}]},]

db.collection.aggregate([{$unwind:"$subs"},{$match:{"subs.subsub":{$ne:null}}},{$group:{_id:"$subs.subsub.a"}},{$project:{a:"$_id",_id:false}}])