On Fri, 16 Feb 2001, you wrote:
> > I also fail to understand why it is a problem. How is recursive
> references
> > different to any other entity relationship? Just store a PK, a Handle,
> > whatever...
>
[snip]
> Now an example of what's been bamboozling me.
>
> Suppose the taxonomy were of species; Butterfly would be a subclass of
> Insect, which in turn subclasses Animal, likewise Organic. Consider a
> particular butterfly called Jim, represented by a row in the Thing table.
>
> We assert that Jim is a butterfly by creating a suitable row in the
> ThingCategory table. However, because Butterfly is a subcategory of Insect
> (etc), by implication Jim is a member of the classes insect, animal and
> organic. Inheritance.
>
> Certainly it is possible to materialise the categories that apply by
> implication to Jim (or any other thing) but I am somewhat at a loss to say
> how one might (efficiently) go about retrieving a set of Things that are
> members of the category Animal.
Ew, yuck, yeah, I see what the problem is. How would you express it in SQL?
Stored procedure, perhaps...
Otherwise, you'll need to construct an inheritance tree somehow. If you *know*
that EJBs will be your only client, you could construct an RMI cache of the
inheritance tree at startup, and then access that from your bean. This would
be foul to try to maintain if the database was being updated externally, though.
Perhaps the best way would be to have a Category bean. The home interface has
a create(...) method, a findByPrimaryKey(...) method, a findByName(...) method
etc. The remote interface has probably only two methods, getSubCategories()
and getMemberThings(). Then, in a finder somewhere, you get the Category for
the type you want, call getMemberThings() on it, add them to a Vector, then
recursively call getSubCategories(), calling getMemberThings() on each one, and
adding them to the Vector().
I don't know how this would perform; my suspicion is that it would get rapidly
worse as the network got more complex, and would of course not handle circular
references and the like.
My $0.02
Tom
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