[{"_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:{children:{$map:{input:"$children",as:"a2",in:{$mergeObjects:["$$a2",{$cond:[{$eq:["$$a2.name","l2a"]},{name:"l2g",route:{$concat:["$name","/","l2g"]},children:{$map:{input:"$$a2.children",as:"a3",in:{$mergeObjects:["$$a3",{route:{$concat:["$name","/","l2g","/","$$a3.name"]}}]}}}},{}]}]}}}}}])