Question

I m mapping a proprietary database to Hibernate for use with Spring. In it, there are a couple of jointables that, for entity A and entity B have the following schema:

CREATE TABLE AjoinB (
  idA int not null,
  idB int not null,
  groupEnum enum ( groupC ,  groupD ,  groupE ),
  primary key(idA, idB, groupEnum)
);

As you can see, this indicates that there can be multiple A-B relationships that put them in different groups. I d like to end up with, first line for entity A and second for entity B, the following sets

Set<B> BforGroupC, BforGroupD, BforGroupE;
Set<A> AforGroupC, AforGroupD, AforGroupE;

So far, I ve only managed to put them in one set and disregard the groupEnum relationship attribute:

@ManyToMany(targetEntity=B.class, cascade={ CascadeType.PERSIST, CascadeType.MERGE } )
@JoinTable(name="AjoinB", joinColumns=@JoinColumn(name="idA"), inverseJoinColumns=@JoinColumn(name="idB") )
private Set<B> BforAllGroups;

and

@ManyToMany( mappedBy = "BforAllGroups", targetEntity = A.class )
private Set<A> AforAllGroups;

How can I make multiple sets where they belong either in groupC, groupD or groupE?

Cheers

Nik

Answer 1

If you re considering doing this, don t. Tables are cheap nowadays what s with the economy and all, so just create one per association; it ll be so much easier.

If you re bound by a legacy database and you can t change the structure of that table I would

Consider skaffman s solution first (+1, btw). Depending on your target database you may be able to write a trigger for your views that would insert adequate "discriminator" value.
If the above isn t possible in your DB, another solution is to use custom SQL for CRUD operations for your collections. Keep in mind that this will NOT work (e.g. your "discriminator value" won t get applied) for complex HQL queries involving your association as part of condition. You can also mix / match this with above - e.g. use views and use custom SQL for insert / delete.
If both of the above fail, go with "association as a separate entity" as suggested by framer8. That s going to be rather ugly (since we re assuming here you can t change your tables) due to composite keys and all extraneous code. It may, in fact, be impossible if any of your associations allows duplicates.

Answer 2

To my knowledge, Hibernate cannot use such a "discriminator" column in the way that you want. Hibernate requires a join table for each of them.

Perhaps you might be able to define additional views on the table, showing each of the groupings?

Answer 3

I think the advise anytime you need to access a field in a link table is to make the link table an object and a hibernate entity in its own right. A would have a set of AtoB objects and AtoB would have a set of B objects. I have a simmilar situation where the link table has a user associated with the link.

select joinTable.b from A a 
left join a.AtoB joinTable
where joinTable.group =  C

It s not as elegant as having an implicit join done by hibernate, but it does give you the control you need.

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