How to add an object to existing List using Spring Mongo db? -

this class a

@document class user{ private string id ; private string name; @dbref private list<socity> socitys; } 

and class socity

@document class socity{ private string id ; private string name; } 

and add user function

public user adduser(user user) {     list<socity> socity = new arraylist<>();     user.setsocitys (socity );     return  userrepository.save(user); } 

i want add socity existing user try doesn't work

public static void main(string[] args) {     applicationcontext ctx  = springapplication.run (app.class, args);     socitydao sdao = ctx.getbean(socitydao .class);     userrepository userrepository = ctx.getbean(userrepository.class);      user u = userrepository.findone("");     socity s = new socity("soc1");     sdao .addsocity(e);     u.getsocitys().add(e);  } 

this rest service

@requestmapping(value = "up/{id}", method = requestmethod.post ,produces =  "application/json") public user addsocitytouser(@requestbody socity, @pathvariable  string id)  {       return socitydao.addsocitytouser(e, id); } 

in end of code add userrepository.save(u) persist changes db.

as long as id (because persisted object) updated. if has no id saved new object in db.