[{"_id":"1","attrib_1":"value_1","attrib_2":"value_2","months":{"2":{"month":"2","year":"2008","transactions":[{"field_1":"val_1","field_2":"val_2",},{"field_1":"val_4","field_2":"val_5","field_3":"val_6"},]},"3":{"month":"3","year":"2018","transactions":[{"field_1":"val_7","field_3":"val_9"},{"field_1":"val_10","field_2":"val_11",},]},}}]

db.collection.aggregate([{"$addFields":{"latest":{"$map":{"input":{"$objectToArray":"$months",},"as":"obj","in":{"all_field_1":{"$ifNull":["$$obj.v.transactions.field_1",[""]]},"all_field_2":{"$ifNull":["$$obj.v.transactions.field_2",[""]]},"all_field_3":{"$ifNull":["$$obj.v.transactions.field_3",[""]]},"all_months":{"$ifNull":["$$obj.v.month",""]},"all_years":{"$ifNull":["$$obj.v.year",""]},}}}}},{"$project":{"_id":1,"months":"$latest.all_months","year":"$latest.all_years","field_1":"$latest.all_field_1","field_2":"$latest.all_field_2","field_3":"$latest.all_field_3",}}])