[{"_id":ObjectId("5a934e000102030405000000"),"name":"l1","route":"l1","children":[{"name":"l2a","route":"l1/l2a","children":[{"name":"l3a","route":"l1/l2a/l3a"}]},{"name":"l2b","route":"l1/l2b","children":[{"name":"l3b","route":"l1/l2b/l3b"}]}]}]

db.collection.aggregate([{$match:{_id:ObjectId("5a934e000102030405000000")}},{$set:{name:"l4",route:"l4",children:{$map:{input:"$children",as:"a2",in:{$mergeObjects:["$$a2",{route:{$concat:["l4","/","$$a2.name"]},children:{$map:{input:"$$a2.children",as:"a3",in:{$mergeObjects:["$$a3",{route:{$concat:["l4","/","$$a2.name","/","$$a3.name"]}}]}}}}]}}}}}])