db={"mes":[{"_id":1,"mes":"ENERO"},{"_id":2,"mes":"FEBRERO"},{"_id":3,"mes":"MARZO"}],"ventas":[{"_id":1,"detalles":[{"_id":1,}]},{"_id":2,"detalles":[{"_id":3,}]},{"_id":3,"detalles":[{"_id":3,}]}]}

db.mes.aggregate([{"$unwind":"$_id"},{"$lookup":{"from":"ventas","localField":"mes._id","foreignField":"ventas.detalles._id","as":"trans"}},])