> 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...
In a schema I have a Category table related m:m to itself via a Subcategory
table, allowing the recursive construction of taxonomies of indeterminate
depth. This is the self-referential part (what I called a recursive
structure).
There is also a Thing table, and a ThingCategory table that allows things to
be assigned any number of classifications.
It is possible for categories to apply to a thing indirectly by implication
of what might be described as taxonomic inheritance. (In fact the taxonomy
is potentially a network rather than an hierarchy, so there can be multiple
inheritance, but that is another matter.)
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.
The problem, as I see it, is primarily one of inability on my part to
express the above ideas in EJB terms. It is with the semantics of the model
that I most want help. On re-reading the EJB tome currently ballasting my
desk, I now suspect that this will be done with finder methods. I am in the
process of coding a utility class that does the recursive expansion of
categories given a starting category, in order to succinctly express the
finder method.
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