OneToMany Unidirectional relationship without Join table (JSR 220)

[Rahul Juneja]

Folks,

I was just going through JPA Specs(JSR 220) and this doesn't seems to support Unidirectional relationships with two tables until and unless we have a join table in between.

Do you think i have any other alternative if i don't want to create a join table or don't want to have a bidirectional relationship.

Thanks,
Rahul

[Edvins Reisons]

Yes, according to the specification, a join table is the default implementation, and other implementations are optional.

[Cameron Wallace McKenzie]

Here is a sample tutorial that uses Hibernate and JPA annotations to map a bi-directional one-to-many and many-to-many relationship, and there is no join table needed:



Mapping One-to-Many and Many-to-One Bi-directional Relationships



Now, for a many-to-many relationship, you need that join table, but I don't see the need on basic one to one and one to many relationships.

[Edvins Reisons]

Cameron,

I agree with your "I don't see the need..." statement, and yet this is what the specification says (from p. 9.1.24):

The default schema-level mapping for unidirectional one-to-many relationships uses a join
table, as described in Section 2.1.8.5. Unidirectional one-to-many relationships may be implemented
using one-to-many foreign key mappings, however, such support is not required in this
release. Applications that want to use a foreign key mapping strategy for one-to-many relationships
should make these relationships bidirectional to ensure portability.

[Rahul Juneja]

Cameron,

Thanks for the help, but i guess i am looking for unidirectional without the join table.

Thanks,
Rahul

[Tim Holloway]

What is your JPA implementation? The Apache OpenJPA system does support joins without a join table and I've been using it extensively for some time.

In fact, I'd expect most serious JSR-220 implementations to support table-less joins for the simple reason that too many legacy databases were designed with direct foreign key linkages.

[Rahul Juneja]

Tim,

You are right we are stuck in a situation with some old legacy database without a join table.

That does make sense to have a unidirectional without the join table.

Thanks,
Rahul