Re: many-to-many relationship

At 05:20 PM 10/8/2008, pgsql-sql-owner(at)postgresql(dot)org wrote:
>Date: Wed, 8 Oct 2008 11:25:10 +0200
>From: Louis-David Mitterrand <vindex+lists-pgsql-sql(at)apartia(dot)org>
>To: pgsql-sql(at)postgresql(dot)org
>Subject: Re: many-to-many relationship
>Message-ID: <20081008092510(dot)GA23361(at)apartia(dot)fr>
>Mail-Followup-To: pgsql-sql(at)postgresql(dot)org
>References: <20081007132118(dot)C80641E81548(at)mx2(dot)hub(dot)org>
><20081008001705(dot)88A7B5B93F8F9(at)zenon(dot)apartia(dot)fr>
>In-Reply-To: <20081008001705(dot)88A7B5B93F8F9(at)zenon(dot)apartia(dot)fr>
>X-Archive-Number: 200810/23
>X-Sequence-Number: 31665
>
> >
> > |id|image_url|f_table|f_key
> > |1 |url......|person |1234
> > |2 |url2.....|event |5678
> >
> > I think this is called a "polymorphic join" but I could be wrong
> about
> > that. I'd guess you could construct a rule or trigger to validate
> the
> > foreign key data on insert/update but that's out of my skill area.
>
>Hi Steve,
>
>So in your solution the f_table column is just text which needs to be
>validated by a custom trigger?

Hi,

Yup - that's exactly what I'm suggesting. Storing the text value of the
related tables right in the table in question. It might seem insane,
but in my experience it works out reasonably well. Ruby on Rails has
popularized the approach, using it both in the data backend, as well as
in the OO frontend (so Rugy object class to be instantiated is chosen
by the text value of "f_table" for a given row - hence the
polymorphism).

http://wiki.rubyonrails.org/rails/pages/UnderstandingPolymorphicAssociations

There are some situations where this approach could create problems but
if in general all you're doing is select statements along these lines:

select * from images where
f_table = 'person' and f_id = '1234'

There's not much to go wrong. (Famous last words).

And regarding the custom validation by trigger, I'd think that would
work just fine. I'm not an expert on triggers, rules and constraints in
Pg though. (I do all my validation in the middleware, which might give
some people here high blood pressure). :)

Keep us posted on which solution you choose and how it works out for
you!

Steve