[Modeling-users] Re: Money and Modeling

[Sebastien B. <sbi...@us...>]

Yannick Gingras <yan...@sa...> wrote:
> What are you guys doing with monetary values ?
>=20
> mx.DateTime is a really nice class for dates management. Is there a class
> out there for money ?
>=20
> I'm not certain I fully understand how Modeling treats the SQL decimal
> type.  Decimal should be fixed points but retrieving and attribute
> defined by :
>    <attribute displayLabel=3D''
>               scale=3D'4'
>               name=3D'currencyRate'
>               isRequired=3D'1'
>               externalType=3D'numeric'
>               defaultValue=3D'None'
>               isClassProperty=3D'1'=20
>               precision=3D'10'=20
>               columnName=3D'currency_rate'
>               type=3D'string'
>               width=3D'0'/>
> gives a float which is not quite fixed point...  Is there anything wrong
> with my model ?

  (It could have helped to know what you are getting, and what you were
  expecting --I simply assume that this was an approximation problem, if
  not, please be more explicit).

As Soaf said:
> Fixed point doesn't exist in the default python so . You always retrieve a
> numeric but it's map to a float in python.  And please be carefull about
> this as, python do some strange round while using print() and stuff like
> that .

--> The problem here is python combined with the adaptor, this is not a
problem with the framework in itself. Simply look at this (using the
test package AuthorBooks):

>>> import psycopg
>>> cnx=3Dpsycopg.connect(dsn=3D"host=3Dlocalhost dbname=3DAUTHOR_BOOKS use=
r=3Dpostgres")
>>> cur=3Dcnx.cursor()
>>> cur.execute("INSERT INTO BOOK(id,title,price) VALUES(5,'Test Title',7.9=
2)")
>>> cnx.commit()
>>> cur.execute('SELECT * FROM BOOK WHERE id=3D5')
>>> cur.fetchone()
(7.9199999999999999, 'Test Title', 5, None)

Since your column is NUMERIC(10,4), any python adaptor (all, but sqlite
for which all types are strings) will return a float, and there's
nothing like fixed point float in python. So you'll get a value
inherently dependent to the binary representation of the value, see:

>>> 7.48
7.4800000000000004
>>> 7.85
7.8499999999999996
>>> 7.85=3D=3D7.8499999999999996
1

  However, I noticed that your python type is _string_, I presume this
  is intended. What if you store the value in a VARCHAR? You'll stay
  away from these problems, then. BTW, have you looked at the python
  cookbook or searched for any monetary handling package? I'm quite
  confident such problems must have been already addressed somewhere,
  it's probably worth the search.

        Regards,

-- S=E9bastien.