Re: [SQLObject] Composed Primary key

[Ian B. <ia...@co...>]

On Tuesday, August 5, 2003, at 05:59 PM, Magnus Lyck=E5 wrote:
> order_line
> |id |order_id|spec          |price_amt|price_currency|
> +---+--------+--------------+---------+--------------+
> |  1|     123|Dog collar    |12.50    |EUR           |
> |  2|     123|Dog food      |0.34     |USD           |
>
>
> In this case you could say that the price is a value from a
> design point of view, but an object from a programming point
> of view. Martin Fowler calls such objects "Value Objects" in
> the book "Patterns of Enterprise Application Architechture".

Indeed, any value object (i.e., immutable) will work fine with=20
SQLObject, using a certain idiom:

class OrderLine(SQLObject):
     priceAmt =3D CurrencyCol()
     priceCurrency =3D String(length=3D3)

     def _get_price(self):
         return Price(self.priceAmt, self.priceCurrency)
     def _set_price(self, value):
         self.set(priceAmt=3Dvalue.amount, priceCurrency=3Dvalue.currency)=


class Price(object):
     def __init__(self, amount, currency):
         self.amount =3D amount
         self.currency =3D currency
     # and probably more methods too


It is important that Price be immutable (or at least treated that way),=20=

as the OrderLine instance won't detect changes to a Price instance, so=20=

they won't be saved in the database.

In fact, Price instances could be mutable if the instances were also=20
given a reference to the containing OrderLine, and notified it of=20
changes.

The whole idiom could be encapsulated in something like a=20
compositeproperty (analogous to property), but that would be only=20
slightly more compact while being less clear, IMHO.

I think this idiom is sufficient, so I don't plan any other support. =20
Except for the primary key, which is somewhat unique in that SQLObject=20=

uses that column internally.

   Ian