Re: [SQLObject] Decimal type support

[Carlos R. <car...@gm...>]

On Mon, 06 Dec 2004 10:15:34 -0600, Ian Bicking <ia...@co...> wrote:
> Carlos Ribeiro wrote:
> > I just downloaded & installed the decimal module (the same from Python
> > 2.4) for Python 2.3. I know that SQLObject has a DecimalCol type. I've
> > tried to check the code that actually fetches decimal values from the
> > DB, but I got a little bit lost on SQLObject internals at this point.
> > I trie to grep for Decimal and found only a few references, so I
> > assume it's not full implemented yet.
> 
> One way would be to handle the conversion through the driver.  This
> should be doable with psycopg.  Some way you have to get the value from
> the database, probably as a string, then convert it to decimal.
> Obviously it's not very helpful if you get it from the database as a
> float, then convert it to a decimal.
> 
> Right now there's no good hooks for coercing something before it is
> returned.  E.g., in Postgres I think that'd be something like "SELECT
> (CAST dec_column AS TEXT) AS dec_column", which would keep psycopg from
> converting to a float.

Humm. Decimals should always be passed around either as native
decimals or as strings; the Decimal constructor takes a string as a
parameter, and that's the only way to preserve precision. There are a
few ways out of this problem:

1) Rely on the DB driver working with native decimals (perhaps with
some patches, as you suggested, to trick the DB to use a string as an
intermediate value). That's a definitive solution but I'm not sure
even how to start it.

2) Forget decimals on the DB -- store everything as strings, and
convert to native Decimals on reading & writing. This works, but in
this case I can't rely on the DB itself for any arithmetic
manipulation. That will cause problems with aggregate SQL clauses
(SUM, MAX, etc.).

I really don't know what to do. Implementing Decimal support in
SQLObject would be nice, but I fear that it will take a lot of time
and require a lot of knowledge about individual database drivers. What
a problem...


-- 
Carlos Ribeiro
Consultoria em Projetos
blog: http://rascunhosrotos.blogspot.com
blog: http://pythonnotes.blogspot.com
mail: car...@gm...
mail: car...@ya...