[{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([{"$project":{_id:0,ProjectId:1,Extras:{$concatArrays:[[{Billings:"$Billings"}],"$Extras"]}}},{$unwind:"$Extras"},{$unwind:"$Extras.Billings"},{$addFields:{"Extras.Billings.ExtraId":"$Extras.ExtraId","Extras.Billings.ProjectId":"$ProjectId"}},{$replaceRoot:{newRoot:"$Extras.Billings"}}])