[{"artifacts":[{"_id":0,"a":1},{"a":3},{"a":2}]}]

db.collection.update({},[{"$set":{"artifacts":{"$function":{"body":"function (ar) {return ar.map(x => { if(x.hasOwnProperty('_id')) return x; else {x[\"_id\"]=new ObjectId(); return x;}})}","args":["$artifacts"],"lang":"js"}}}}],{"multi":true})