google app engine - Null value returned while finding entity by key in JPA -

i developing app engine backend android 2 entities: user , item in 1 many relationship. after inserting item entity, receive inserted entity in client app generated key of item converted string , stored in database.later, while trying retrieve same entity passing keystring backend , converting key, getting null entity.what reason?

code retrieve , delete item entity:

@apimethod(name = "removeitembyid")     public item removeitembyid(@named("itemidstring") string idstring) {     //getting key idstring..                key key=keyfactory.stringtokey(idstring);     entitymanager mgr = getentitymanager();     try {         //this line null value returned...         item item = mgr.find(item.class, key);         mgr.remove(item);         mgr.gettransaction().commit();     }     catch(exception e) {         mgr.gettransaction().rollback();         return null;     }     {         mgr.close();     }     return new item(); } 

item entity:

   @entity    public class item {  @id  @generatedvalue(strategy = generationtype.identity)  private key itemid;  private string itemname;   @manytoone(fetch=fetchtype.lazy)  @joincolumn(name="owner_id")  public user owner;   ////getters , setters......   } 

user entity:

    @entity     public class user {      @id     private string id;     private  string username;      @onetomany(targetentity=com.example.xyz.item.class,       mappedby="owner",cascade=cascadetype.all)     private arraylist<item> items;     //getters , setters...     }