[{ProjectId:111,Billings:[{FieldA:1},{FieldA:2}],Extras:[{ExtraId:"E_111_01",Billings:[{FieldA:3},{FieldA:4}]},{ExtraId:"E_111_02",Billings:[{FieldA:5},{FieldA:6}]}]},{ProjectId:222,Billings:[],Extras:[{ExtraId:"E_222_01",Billings:[{FieldA:7},{FieldA:8}]}]}]

db.collection.aggregate([{"$unwind":"$Extras"},{"$addFields":{"Extras.Billings.ExtraId":"$Extras.ExtraId"}},{"$project":{"ProjectId":"$ProjectId","Billings":{"$concatArrays":["$Billings","$Extras.Billings"]}}},{"$unwind":"$Billings"},{"$project":{"ProjectId":"$ProjectId","FieldA":"$Billings.FieldA","ExtraId":"$Billings.ExtraId"}},{"$group":{"_id":{"id":"$_id","FieldA":"$FieldA","ProjectId":"$ProjectId","ExtraId":"$ExtraId"}}},{"$project":{"_id":0,"FieldA":"$_id.FieldA","ProjectId":"$_id.ProjectId","ExtraIs":"$_id.ExtraId"}},{"$sort":{"ProjectId":1,"FieldA":1}}])