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. 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.
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.
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