[Zope] Pointer-to-Object Properties

[Thomas B. Passin]

From: Toby Dickenson <tdickenson@geminidataloggers.com>


>> From: Michael Bernstein [mailto:mbernstein@profitscape.net]
>
>> It would be very beneficial to be able to 'point' to
>> arbitrary objects as if they
>> were sub objects, thus enabling one-to-many, many-to-one, and
>> many-to-many
>> relationships not constrained by the object heirarchy.
>>
>> Classic example: Many author objects, many book objects.
>> Authors can have written
>> several books, and certain books are collaborations with more
>> than one author. It
>> does not make sense for books to be contained within authors,
>> or authors to be
>> contained within books.
>>
>> While this simplistic scenario would allow us to either
>> assign a list property
>> with  book titles to an author object, or a list property
>> containing authors names
>> to a book object, and live with the small amount of
>> duplicated data
>
>I think the key observation here is that the additional information
>should be sufficient to identify the foreign object.... then the
>pointed-to object can be located by searching a Catalog.
>
...

Many-to-many relationships are always tricky.  As others have pointed out in
this thread, it is highly desirable to have some means to maintain data and
referential integrity.  In SQL this is done by declaring create-delete-update
rules for each foreign key relationship.

Also, you want to make keep the properties of the two types of objects in the
many-to-many separate, to minimize coupling.  In the case of books, one way to
do this is to have a book maintain a list of its authors, and each author to
maintain a list of its books.  Then a book and an author only need to know about
lists, not about authors and books.  You can delete a list item without deleting
its parent object.  But now, if a parent object is deleted, how do you make sure
all references to it in various lists are also deleted?  Not so easy!  Back to
data integrity again.

My point is really that object encapsulation and data integrity are very
important, and it is easy to jump to a design that does not provide for them.
If we're going to come up with a Zope-ish way let's make sure its design is well
thought out in these areas.

Tom Passin