Database
[{_id:ObjectId("5f15414ff3812f4838391edd"),"articles":[{"country":"DE","articleNumber":"123456789"},{"country":"AT","articleNumber":"987654321"}]}]
Query
db.collection.aggregate([{$match:{_id:ObjectId("5f15414ff3812f4838391edd")}},{$project:{_id:1,articles:{$map:{input:"$articles",as:"article",in:{country:"$$article.country",articleNumber:"$$article.articleNumber",countryArticleNumber:{$concat:["$$article.country","$$article.articleNumber"]}}}}}}])