Re: [SQLObject] Composed Primary key

[Magnus <ma...@th...>]

At 14:18 2003-07-24 -0400, Brad Bollenbach wrote:
> > Well, if there's going to be composite keys, that might as well be
> > generalized to all the columns (i.e., any attribute could be a composite
> > of several columns).
>
>What's the use case for this?

For instance money objects that consist of an amount and a currency.
You could make them full objects with an identifier and do:

order_line
|id |order_id|spec          |money_id|
+---+--------+--------------+--------+
|  1|     123|Dog collar    |       1|
|  2|     123|Dog food      |       2|

money
|id |amt    |currency|
+---+-------+--------+
|  1|12.50  |EUR     |
|  2|0.34   |USD     |

But is seems simpler to just do:

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".



--
Magnus Lycka (It's really Lyckå), ma...@th...
Thinkware AB, Sweden, www.thinkware.se
I code Python ~ The Agile Programming Language