Hi Aristedes, On May 30, 2007, at 10:09 PM, Aristedes Maniatis wrote:
On 31/05/2007, at 1:03 PM, Craig L Russell wrote:While it might be possible theoretically to define a different column in the database to be used as the association column to join rows of a subclass and a superclass table, by far the most common and most understandable way to map inheritance is to simply assume that the primary key of both tables is the same and that the id field in the class contains the value to be used for both primary keys. The direction of the foreign key constraint should indicate which is the subclass.Yes, 99% of the time this is correct. However, if we want to support compound primary keys there is a need to describe how the columns in the subclass map to the columns in the superclass.
If it really is an inheritance relationship and not a one-one or one- many relationship, the compound primary keys should also correspond exactly.
That way Cayenne can populate them both correctly. Our original plan was to use an object relationship description in the DataMap to describe this pairing. The alternative is to use the names of the object attributes in order to match the primary key columns to each other, but something about this bothers me as an approach.
The most common case I've seen has a foreign key constraint defined on the primary key columns of the subclass table to the corresponding primary key columns of the superclass table.
Also, as Lachlan points out, this means that we don't get to specific nullify, cascade, etc delete rules. If you have a concrete superclass, you may wish to nullify the relationship when deleting the subclass record. Naturally if the superclass is abstract this is not allowed. But specifying the objrelationship explicitly allows us to put these rules somewhere and remove any ambiguity from compound key relationships.
This seems like an implementation detail (which I am very obviously not competent to comment on).
Regards, Craig
Ari --------------------------> Aristedes Maniatis phone +61 2 9660 9700 PGP fingerprint 08 57 20 4B 80 69 59 E2 A9 BF 2D 48 C2 20 0C C8
Craig Russell Architect, Sun Java Enterprise System http://java.sun.com/products/jdo 408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!
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