Form data transition into entity beans to persist them with the server side ORM

Question

Is there any good explanation or tutorial which describes the common ways to create entity beans from the received data of a form? The main reason for my question the treating the received ids (e.g id of country,city and so forth) which is the way from the id to entity?

Example:

................Client side form 

username:String
countryid:Integer (could be a drop down)

................Server side entities

public class UserBean
{
  String username;
  CountryBean Country;
}


public class CountryBean
{
  String cityname;
  Integer id;
}

............................................

Maybe the question is dependent on what technology is used, but I guess there is a very common way. I would like to comprehend the conventional approaches to this problem. (For the sake of the completeness I would like to save the form data (received by Stripes) with JPA)