[ZODB-Dev] PROPOSAL: ZODB Relationships

[Jeremy Hylton]

On Thu, 2003-05-08 at 17:07, roche at upfrontsystems.co.za wrote:
> We finally have a proposal out for ZODB relationships. This proposal
> presents an API for relationships, summarises ideas and contributions
> from a lot of people and was fuelled by the recent discussions about
> relationships on Zope3-dev and Zope-dev.
> 
>     http://www.zope.org/Members/upfront/ZODBRelationships
> 
> Your comments would be appreciated.

I have a few suggestions about the "providing objects with views of
relationships" section.  

My personal interest is in objects that store the relationships
internally rather than in an external data structure.  This seems like a
natural structure for a ZODB application, because it keeps the
references local to the objects involved; you don't need to load some
external objects to access the relationship.  This isn't the right
approach for all applications, but I think it's useful in many cases.

How do we implement this given the current proposal?  I suppose that
there is more than one kind of Relationship object.  An implementation
that provides its own storage must be reachable from the database root
explicitly.  An implementation that stores relationships on the objects
is only used at runtime to associate storage with other persistent
objects.  Is that right?  

The proposal should clarfy how implementations might reasonably store
the relationships.  If an implementation uses a BTree, it would be
necessary for the objects in the relationship to be totally ordered.  If
the implementation uses a dictionary, then they would need to define an
__cmp__() and an __hash__() or neither.  It might be useful if the user
could provide a hint to the implementation about what is acceptable for
these objects.

I would like to see a different approach for specifying cardinality,
because there are really only two possible values -- "single" and
"multiple."  (Might one and many be more consistent with the typical
jargon?)  Rather than specifying a magic string as a keyword argument,
why don't we have to different RelationshipView classes?

I think the word "View" is probably carries too much baggage in Zope. 
I'd rather find another word for the "end" of the relationship.

I also wanted to comment on the risk factors.  I think the
implementation should start in ZODB4, where it would actually be part of
the persistence package.  We expect to migrate Zope 2.x to ZODB4
sometime this summer, so I wouldn't worry too much about ZODB3 support.

Jeremy