[{"_id":"top1","title":"face","depth":0,"parent_category_ids":[],"child_category_ids":["middle1","middle2"]},{"_id":"middle1","title":"eyes","depth":1,"parent_category_ids":["top1"],"child_category_ids":["third1"]},{"_id":"middle2","title":"mouth","depth":1,"parent_category_ids":["top1"],"child_category_ids":["third2"]},{"_id":"third1","title":"eyelid","depth":2,"parent_category_ids":["middle1"],"child_category_ids":[]}]

db.collection.aggregate([{$lookup:{from:"collection",localField:"child_category_ids",foreignField:"_id",as:"child_categories"}}])