On May 30, 2007, at 7:23 PM, Lachlan Deck wrote:
On 31/05/2007, at 11:54 AM, Lachlan Deck wrote:On 31/05/2007, at 10:44 AM, Mike Kienenberger wrote:On 5/30/07, Aristedes Maniatis <[EMAIL PROTECTED]> wrote:When we get to vertical inheritance there will be at least one other property added to the ObjEntity: the name of the relationship used tofind the superclass.When we get to inheritance, we'd be wise to follow the JPA methodologyfor specifying inheritance relationships. Vertical and single-table is well-defined.<...>Okay, so consider the request for superRelationship dropped. (See disclaimer :-) Well that makes it simpler to do all of this ;-)Hang on... Hibernate, for example, does indeed utilise a relationship, to my understanding [1], that specifies the nature of the relationship to the parent, what happens when an object is deleted and so on. These are characteristics that are encapsulated in a relationship. This is why the request for defining a superRelationship in ObjEntity was first made. Perhaps it's an optional for vertical where in its absence is some implicit strategy.Thoughts?
The example below shows in Hibernate how to remove the automatically generated relationship between Person and Employee due to the need to map the primary key association to inheritance.
I'd even call this a deficiency in the tool, since there is a foreign key constraint defined on the primary key column of the employees table that refers to the primary key column of the persons table. The most natural mapping of this would be inheritance and should therefore be the default.
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.
Craig
with regards, -- Lachlan Deck [1] http://www.warfrog.com/hibernatetutorial2/
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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