db={test:[{"_id":ObjectId("5efc8285d573109c093229fe"),"units":[{"keyword_id":"44","rating":{"exact_rating":{"rating":"55"}}},{"keyword_id":"45","rating":{"exact_rating":{"rating":"43","sample-field":"Yes"}}},{"rating":{"exact_rating":{"rating":1,"sample-field":"No"}}}],"result_id":111,}]}

db.test.aggregate([{$addFields:{units:{$map:{input:"$units",as:"unit",in:{$mergeObjects:["$$unit",{rating:{exact_rating:{rating:{$convert:{input:"$$unit.rating.exact_rating.rating",to:"int",onError:{badRating:"$$unit.rating.exact_rating.rating"}}}}}}]}}}}}])