Re: Inheritence woes

On 24 Apr 2003, Hadley Willan wrote:

> We've been playing with Inheritence and have perhaps not used it
> correctly.
>
> Our original intention was to have a generic table called unit, then we
> dealing with different types of units, like boats, or cars, or marine
> outboards for example, they can then inherit off unit and get a common
> set of attributes.
>
> As we've progressed things like the purchase costs for these items needs
> to be recorded and we've added in a table like, unit_purchase_cost.
> However, because the unit type car inherits unit, unit_purchase_cost
> can't use a foreign key that references unit.id because the values are
> in car.
>
> Doing a SELECT * FROM ONLY unit; shows no values as they're all in car.
>
> About the only away around this right now we can see is to write our own
> foreign key triggers, allowing the insert of the value without the
> referential integrity to the parent class?
>
> Any other thoughts on this?

Inheritance is a mess? ;)

Seriously, you'll need to do some sort of workaround (or replacement) for
the foreign key triggers, you may need to do a workaround on the primary
key/unique constraint on id (I figure there must be one since otherwise
the foreign key wouldn't work) since that won't prevent you from inserting
a car with id 1 and then a boat with id 1 (if you use a sequence and never
insert manually you don't have to worry about this as much).