how to manage jpa bidirectional relation ships correctly? - jpa

Question

I am using EJB3.2 and JPA 2.1.
In my application I have these entities (in brief) :
#Entity
public class Festival
{
int Id;
String name;
...
#ManyToOne
#JoinColumn(name = "merchant", referencedColumnName = "Id")
Merchant merchant;
}
#Entity
public class Merchant
{
int Id;
String name;
...
#ManyToOne
#JoinColumn(name = "category", referencedColumnName = "Id")
Category category;
#ManyToOne
#JoinColumn(name = "city", referencedColumnName = "Id")
City city;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "festival")
List<Festival> festivalList;
}
#Entity
public class Category
{
int Id;
String name;
...
#OneToMany(cascade = CascadeType.ALL, mappedBy = "merchant")
List<Merchant> merchantList;
}
#Entity
public class City
{
int Id;
String name;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "merchant")
List<Merchant> merchantList;
}
thus, (Festival, Merchant) has a bidirectional relationship like (Merchant, Category) and (Merchant, City)
the problem is when i remove or change a merchant, still there is an unchanged copy of it in categories merchantList and cities merchantList ! and so on..
how should I manage these changes?! why JPA doesn't do the changes to other copies it self? doesn't it increase the wrong copies risk?!

Answer 1

JPA treats your entities like regular java objects and does not maintain relationships for you - the application is responsible for ensuring that both sides of a bidirectional relationship are kept in sync when you change one side. So when you dereference a City from a Merchant, you must remove the reference to the Merchant from the City's merchantList and then merge if necessary.
Otherwise your view of the data via the java objects will become out of synch with the database until the objects are refreshed from the database. You can weigh the value and costs of keeping both sides in sync or refreshing when needed or even not mapping the non-owning side and determine what is better for your application on an entity by entity basis.

Answer 2

Make sure to persist(merchant) before calling categories.getMerchantList().

Answer 3

I think this quote from "Java Persistence With Hibernate" (p. 261) by Christian Bauer and Gavin King may be useful for you :
Contrary to EJB 2.0 CMR, Hibernate and JPA associations are all
inherently unidirectional.
So the association from Merchant to Category is different than association from Category to Merchant, and you need to manage them separately.
edit:
I adjusted your initial bean config to look as it should:
(Of course, you will use an appropriate primary key generation strategy.)
#Entity
public class Festival
{
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
int Id;
String name;
#ManyToOne(cascade=CascadeType.ALL)
#JoinColumn(name = "merchant")
Merchant merchant;
}
#Entity
public class Merchant
{
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
int Id;
String name;
#ManyToOne
#JoinColumn(name = "category")
Category category;
#ManyToOne
#JoinColumn(name = "city")
City city;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "merchant")
List<Festival> festivalList;
}
#Entity
public class Category
{
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
int Id;
String name;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "category")
List<Merchant> merchantList;
}
#Entity
public class City
{
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
int Id;
String name;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "city")
List<Merchant> merchantList;
}