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

[Phillip J. Eby]

At 10:38 AM 5/9/03 +0100, Chris Withers wrote:
>Roché Compaan wrote:
>>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

Yes, directed graph, sorry.  I rarely encounter undirected graphs, outside 
the discussion of this proposal.

In real life, *most* relationships are directed.  Your examples in the 
proposal are all directed relationships, that just happen to have non 
overlapping link-end sets.  (That is, a course can't be a student or vice 
versa.)


>>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?
>
>You can only use containment once ;-)
>
>I can't think of any good examples, but if you wanted to express more than 
>one relationship that wasn't direction-agnostic, then you couldn't use 
>containment for both of them.

What Chris said.  :)  Also note that using containment to implement a 
directional relationship requires the object to know about the 
relationship, so that violates your expressed requirement that you be able 
to create relationships between objects that don't know the relationship exists.