[Zope3-dev] Vocabularies, sources and relationships

[Gary Poster]

On Apr 6, 2005, at 4:57 AM, Santi Camps wrote:

> Hi all,
>
> I'm just beginning to study Zope 3 (helped by the magnificent Philipp 
> von Weitershausen's book).   I'm specially interested in the schema 
> framework, and how it can be used to implement relationships between 
> objects (if it can be used that way).

It can be and has been.

> I know that are proposals to replace vocabularies by sources, as 
> described in the thread:
> http://mail.zope.org/pipermail/zope3-dev/2004-September/012096.html
>
> My doubs are:
> 1) Are sources already implemented ?  If not, it will be ?

Basic sources are implemented.  Several additional features need to be 
implemented for them to be able to fully replace vocabularies.  Off the 
top of my head, I believe that sources now have widgets that support 
searching but not iterating; vocabularies support both.  Improving 
sources to be a full-fledged replacement for vocabularies is an 
important TODO.  We are currently using sources when possible, and 
using vocabularies otherwise.

> 2) In any case, are sources/vocabularies a good way to implement 
> relationships between objects ?

They can certainly model them, and they are the only reasonable way to 
do it in schema of which I am aware.

The storage of the relationship--the implementation--is another 
decision, which we happened to just talk about around here.  Standard 
python references?  Persistent weak references?  Descriptor with an 
external data source, like the schoolbell relationship tool, as a back 
end?  They all have pros and cons, some of them subtle.

> I think an implementation of ISource and ITerm/s and so on could be 
> writted providing values obtained from other objects, but I'm not sure 
> what could happen with this relationship after created.   For 
> instance, if the referenced object is deleted after that.    Is there 
> any utilities in Zope 3 to take care about referential integrity 
> between objects ?
> In Zope 2, I've added some Foreign Key capabilities to Z Catalogs, 
> using one catalog per meta_type and overriding manage_beforeDelete and 
> so on.  So, for instance, a customer can't be deleted if it has some 
> referencing invoice.  I just one to know if there are better ways to 
> obtain the same result in Zope 3.

The best way, maybe barring the schoolbell tool, to get referential 
integrity right now is probably subscribers.  There are some other 
frameworks in the repository to help with this--Jim mentioned 
dependables--but my understanding is that they are still in an 
experimental state.

Subscribers aren't perfect for this: getting a reasonable UI with 
after-the-fact complaints (as most subscribers will tend to be) is 
sometimes a bit tricky (another use case for ZODB save points; you can 
do stuff like this in a somewhat rickety way--and I have--with 
subtransactions for now), for instance.  They should be workable for 
many examples, though.

Gary