On Tuesday 07 April 2009 05:52:22 Richard Smith wrote
(edited to correct top-posting):
> On Mon, Apr 6, 2009 at 6:15 PM, Malcolm Tredinnick wrote:
> >
> > On Mon, 2009-04-06 at 04:53 -0700, Adys wrote:
> > > I made a small patch for my local Django install to be able to precise
> > > add_constraint=False (default True, normal behaviour) to FKeys/
> > > M2Ms/...
> > > As the tin says, it prevents index/constraint creation for that field.
> > > I won't bother going through the use cases for this; I know I need it
> > > and maybe other people too.
> >
> > No, actually the only way something like this is ever going to get into
> > Django is precisely because there are compelling use-cases.
>
> what would the use cases be?
>
Django relation fields provide two sets of functionalities. One is the
definition of referring columns and the accompanying constraints (and perhaps
indexes); the other is the use of the referring columns in joins, to fetch
related records from the database. It may seem logical to separate the two in
some instances. However, I have yet to see the case where the wish for such
separation is not the result of a database design mistake.
I have run into one example case, where historic (versioned) data was
involved. As SQL support for history of records is nothing to write home
about, mistakes (from a true-relational, theoretic viewpoint) are almost
inevitable; you are forced to choose between evils.
However, since Django is higher-level, I think the solution, even in such
cases, should be with a better abstraction of what you try to achieve, rather
than introducing breakage into foreign keys; generic foreign keys may be a
model for such higher abstractions.
My 2 cents,
Shai.
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