db={productfolders:[{"_id":"62e74dac78c13b738874e1a9","folderName":"Weapons","children":[{"_id":"62e74dd278c13b738874e1ac","folderName":"Bows","parent":"62e74dac78c13b738874e1a9","children":[{"_id":"62e74ddb78c13b738874e1b1","folderName":"Long Bows","parent":"62e74dd278c13b738874e1ac","children":[],},{"_id":"62e74de278c13b738874e1b7","folderName":"Short Bows","parent":"62e74dd278c13b738874e1ac","children":[],}],}]}],products:[{"productName":"Curved Bow","folder":"62e74de278c13b738874e1b7","_id":"62e237368fbde6ed77e3e489"}]}

db.productfolders.aggregate([{$project:{folders:{$function:{body:"function drill(t, n) {if (n.length > 0) {for (let elem of n) {t.push(elem._id); drill(t, elem.children)}} return t};","args":[["$_id"],"$children"],"lang":"js"}}}},{$lookup:{from:"products",localField:"folders",foreignField:"folder",as:"products"}}])