Re: avoid circular references

On Wed, Nov 05, 2003 at 09:15:39PM -0800, Josh Berkus wrote:
> Jerome,
>
> > Each object can contain one or many other objects from the same
> > table, so I guess this relationship can be expressed with something
> > like :
>
> This is called a "tree structure".

Thanks !

> > CREATE TABLE contains (id SERIAL PRIMARY KEY NOT NULL,
> > containerid INTEGER REFERENCES objtable(idobj),
> > contentid INTEGER REFERENCES objtable(idobj) );
>
> This is overcomplicating things, unless a single object can belong to multiple
> containers, which would strike me as peculiar. Easier just to have a
> "containerid" in the objtable, which is your basic Proximity List.
>
> But of course, maybe I'm not understanding you and you do have multiple
> inheritance.

Yes an object could theorically belong to multiple containers, so
that's not really a tree I suppose.

> > What I would like is to be able to avoid circular references :
> >
> > - an object can't contain itself.
> >
> > - an object can't contain one of its containers.
> >
> > So I'd like to know how to create additionnal integrity constraints
> > to solve this problem, and especially what syntax I should use.
>
> Best to use some kind of recursive function. I do this for a calendaring
> setup with event templates, where events can have multiple (possible) parents
> and multiple children. Just write a pl/pgSQL function which reverse-traces
> the parentage of the new object, looking for copies of itself.

Any example for this, especially on how to raise an integrity
exception ?

> > I thought about creating a rule but maybe the best is a trigger
> > (I'm not sure I really understand the difference) if I see
> > what happens when I create new tables. But what can I do in
> > my trigger to have PostgreSQL understand there's an integrity
> > violation ?
>
> A trigger. Just use a BEFORE trigger and raise an exception if a self-parent
> is found.

Thanks for your help !

Jerome Alet
--
"A non-free program is a predatory social system that keeps people
in a state of domination and division, and uses the spoils to
dominate more." - RMS