[{"_id":"62e1f19f094a5696fd18f4e9","parent":null,"children":[{"_id":"44e1f19f094a5696fd18f4o7","parent":"62e1f19f094a5696fd18f4e9","children":[{"_id":"62e1f19f094a5696fd18f4e9","parent":"44e1f19f094a5696fd18f4o7","children":[]}]}]}]

db.collection.aggregate([{$replaceRoot:{newRoot:{$function:{body:"function drill(r) {r.id = r._id; if (r.children.length > 0) { for (let elem of r.children) { drill(elem)}} return r};",args:["$$ROOT"],lang:"js"}}}}])