[Zope] dtml and 'none' behaviour

[garry saddington]

On Friday 11 February 2005 21:21, Chris Kratz wrote:
> You may also use the null= tag on dtml-var.  If the field is None, it will
> replace the value with whatever is in the null="" tag.  Look in the online
> doc that comes with zope.

Thanks this works great. Can you give me the place to find this documented 
because I can't find it?
regards
garry
> ie <dtml-var somefield null="">
>
> We use nulls quite extensively since it bears the connotation of not
> entered which in our case is different then entered but empty.  But, there
> are cases where nulls can throw you if you don't watch it.  One simple
> example is this...
>
> create temp table test (id serial, val text);
> insert into test(val) values('A');
> insert into test(val) values(NULL);
> insert into test(val) values('B');
> select * from test;
>  id | val
> ----+-----
>   1 | A
>   2 |
>   3 | B
> (3 rows)
> select * from test where val<>'A';
>  id | val
> ----+-----
>   3 | B
> (1 row)
>
> Even though this is correct sql, it is hard to get users heads around the
> fact that if you say give me everything where val<>somevalue, it won't pull
> null records.  The correct statement if you do want nulls is:
>
> select * from test where val<>'A' or val is null;
>
> -Chris
>
> On Friday 14 January 2005 03:52 pm, Garry Saddington wrote:
> > Thanks you have confirmed what I thought to be the case - avoid nulls in
> > the database. It is what my instinct told me but I just wanted to have a
> > Zope type view on it.
> > regards
> > garry
> > ----- Original Message -----
> > From: "Tino Wildenhain" <tino at wildenhain.de>
> > To: <garry at scholarpack.org>
> > Cc: <zope at zope.org>
> > Sent: Friday, February 11, 2005 7:34 PM
> > Subject: Re: [Zope] dtml and 'none' behaviour
> >
> > > Am Freitag, den 11.02.2005, 19:21 +0000 schrieb garry saddington:
> > >> I have a zsql method which returns some null values. These are then
> > >> displayed
> > >> in a table but the null values become 'None'. Is there any way to get
> > >> round
> > >> this behaviour and have empty cells in the displayed results? I can do
> > >> it with several dtml-if and dtml-unless calls but wondered if there
> > >> was a more
> > >> subtle approach.
> > >
> > > Well, null is to SQL as None to python. So either you solve this
> > > at database level or in python. For example in python you could
> > > use:
> > >
> > > return [{'foo':row.foo or '',
> > >         'bar':row.bar or ''} for row in context.yourzsqlmethod()]
> > >
> > > with this little "or" you solve this quite elegant. Every
> > > value which evaluates to false (which is: 0, "", ... and None)
> > > let the expression return its right side, which is '' in this
> > > case.
> > >
> > >
> > > On database level you could use coerce() which returns the
> > > first non null argument:
> > >
> > > SELECT coerce(foo,'') as foo,coerce(bar,'') as bar FROM ...
> > >
> > > or even better try to avoid nulls where possible.
> > >
> > > Regards
> > > Tino
> >
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