[ZODB-Dev] Re: [Zope3-dev] PROPOSAL: ZODB Relationships

[Tres Seaver]

On Fri, 2003-05-09 at 05:28, Roché Compaan wrote:
> On Thu, 08 May 2003 19:58:22 -0400
> "Phillip J. Eby" <pje at telecommunity.com> wrote:
> 
> > At 11:07 PM 5/8/03 +0200, 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.
> > 
> > Issues:
> > 
> > 1. "Global relationship repository" has no use cases, and smells like 
> > trouble.  Why not just explicitly store the Relationship() 
> > somewhere?  After all, every RelationshipView will have a reference to 
> > it.  (For ZODB4, a persistent module might be the natural place to put the 
> > "official" reference to the Relationship(); for ZODB3, just hang them off 
> > the root with suitably unique keys.)
> > 
> > 2. There is no directionality to the associations; this doesn't work for 
> > hierarchies or graphs.  E.g. think 'parent_child = Relationship()'.  That 
> > won't work.
> 
> What we have at the moment is a graph and strictly speaking a graph has
> no direction in its edges (maybe you meant directed graph). As far as I
> can see graphs are possible, hierarchies are not. Can't one just use
> containment to represent hierarchies? If one can what other use cases
> are there for directionality in associations?

Dependency:

  - Object 'foo' dependsd on object 'bar'.

Asymmetric business associations

  - Jane is the boss of Fred.

  - Invoice line items decrement the stock count of SKU items.

etc.  I would argue that symmetric (undirected) graphs are much less
common in business applications than asymmetric ones.   

> > All in all, I don't see any reason to make this part of ZODB's API or add 
> > special repositories to support it.  However, like PersistentDict or BTree, 
> > relationships might be a nice tool to have available in the ZODB library.

+1 to that.  I guess I can't figure out what the ZODB has to do with
relationships.

> > A final comment...  Once this takes directionality into consideration, you 
> > might consider calling it an 'Association' rather than a 'Relationship', as 
> > it would then largely meet the MOF and UML semantics of an 
> > "Association".  That is, an association is a collection of directed links 
> > between objects.  It would then make sense to refer to refer to the 
> > 'RelationshipView' as an 'AssociationEnd'.
> 
> We used the term Relationship because the proposal is based on the
> entity-relationship model. I am not too familiar with MOF so maybe you
> can explain how it will apply to the current proposal. Thanks for your
> comments Phillip. I was hoping you say something because you must have
> thought about this when designing PEAK.

E-R models derive largely from the RDBMS world;  UML class models are
not completely isomorphic with them, but might be a better fit for the
kind of thing you are thinking about (for instance, E-R models generated
from class models often show weird "extra" tables to capture the
oddities of mxn relationships).

Tres.
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Tres Seaver                                tseaver at zope.com
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