Posted by Jerry Stuckle on 09/07/05 21:27

Markus Ernst wrote:
>
> Yes, this is a very useful guideline one sometimes tends to forget. Anyway
> in this case the generalized abstract object is useful for handling the
> relations between different things - i.e. a text element can be assigned to
> a page, or to an author (biography), or to a book (abstract). A page can be
> assigned to a rubric, or to an author (extended biography, works list...),
> or to a book (review, excerpt...). Therefore I decided to use one
> centralized ID and relations handling.
>
>

Markus,

But herein lies the problem. Inheritance is not applicable here. A
text element is not a "type of" a page. Rather, a page "contains" one
or more text elements.

A biography is a "type of" a book, so it would be applicable to derive
biography from book. The book also "has an" author, but the author is
not a "type of" text element (or vice versa).

Applying inheritance correctly can be quite complex - and I've seen it
misused quite a bit.

I use inheritance a fair amount. For instance - one I'm working on now
is for a non-profit. I have a hierarchy:

Person -> Member -> Board Member

Person includes non-members who have attended meetings and otherwise
shown an interest in the organization. Members are Persons with a
Member ID number and an expiration date. Board Member has the
additional attribute of Position.

Unfortunately, MySQL isn't an OO DB, so I need to fudge a little. I
could, for instance, put a Position column in the Persons table. But
that would end wasting space for those who are non-members.

So, I assign every person an internal ID (auto increment column). I
have a Members table with the ID, membership number which is linked to
the Person table. And a BoardMember table contains the ID and position.

The beauty of this is that all the work is hidden. If, for instance, I
want to send an email to everyone, I use a PersonList object. For
Members, a MemberList object, and Board Members I create a
BoardMemberList object. *All* the rest of the code to send the emails
is exactly the same. It's just a matter of which object I create. In
fact, the creation of the object is similar to:

switch($POST['mailto']) {
case 'All' :
$list=new PersonList();
break;
case 'Members':
$list = new MemberList();
break;
case 'Board':
$list = new BoardMemberList();
break;
default:
$emsg = 'No list selected';
break;
}
(rest of processing)

This is a perfect *applicable* use for inheritance.


--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
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