Hi, I'm pretty new with Hibernate, and I'm facing some trouble working with composite ids and foreing keys,
I just want to make this work:



And the relation is the following: A has (knows) none ore more instances of B (a list of them), and B has (knows) just one instance of C (this is not a problem by now).

I've read something about using classes to point composite ids (in the official tutorial), overriding equals() and hashCode() but I can't make this work because I'm pretty confused about how to write my hbm.xml files...

Any help? Thanks!

I think you can do this by creating a normal hbm mapping corresponding to Table A, something like the following, assuming all of the columns in the tables are string-valued:



Then for Table B, you could use an hbm mapping like the following:



Sarah

Wait . . . why is atr3 part of the primary key in Table B if it's not part of the primary key in Table A? There's little reason to carry it along into Table B and it breaks some fundamental database design principles. If you don't need it, then you could use the following mappings:

Class A_PK just defines atr1 and atr2 of Table A.
Class A represents all of Table A.
Class B represents all of Table B.



If you do need atr3 to remain in Table B, then I'm not sure you can do much else other than maintain it manually as another <property> in the mapping for Table B. I'd be curious to know if there's a way to do it.

Sarah

Sarah,
Thanks for the response, I'll try it...
atr3 hasn't the same semantics in tables A and B, it could have a totally different meaning... I was just enumerating the attributes in a generic way, maybe I should have used another naming strategy...

I took a look at your requirements more carefully - Table B is a join table between Table A and Table C. I misunderstood that initially. Given that Table A has a composite key, I'm stumped as to how to do it.

I think you want a mapping for Table A as follows that handles the composite key and the non-key column:



The mapping for Table C is straightforward since it has a non-composite key - you can just use an <id> tag for the primary key column.

The question becomes how to do the mapping for Table B. I was thinking you'd do a <composite-id> with a <key-many-to-one> to map the Table A foreign key columns and another <key-many-to-one> to map the Table C foreign key columns. It looks like the <key-many-to-one> syntax allows you to specify multiple nested <column> tags, but I'm not sure how you map the columns into the distinct Java properties.

Sorry to send you down the wrong path with my original response. I misunderstood the table structures.

Sarah