[{"_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([{$project:{months:{$objectToArray:"$months"}}},{$unwind:"$months"},{$unwind:{path:"$months.v.transactions",includeArrayIndex:"index"}},{$group:{_id:{_id:"$_id",year:"$months.v.year",month:"$months.v.month",index:"$index"},fields:{$first:"$months.v.transactions"}}},{$project:{_id:"$_id._id",month:"$_id.month",year:"$_id.year",fields:"$fields"}}])