On Jul 22, 2006, at 4:47 AM, Lorenzo Gil Sanchez wrote:

El vie, 21-07-2006 a las 10:21 -0400, Pete Taylor escribió:
I've run into a very (very!) similar situation, and ended up writing
some wrapper/simplification utilities for zc.relationship.
zc.relationship is wonderfully powerful, but a bit complex.  It's use
is intuitive once you wade through the 2000+ lines of documentation
(which is worthwhile reading), but it's a bit intimidating at first.

Ok, I was afraid of an answer like this because in a recent thread I've
read Jim Fulton saying that we are trying to use zodb as a relational
database which is a shame and I think I agree with that quote.

So that's why I tried not to use external packages like zc.relationship
and stick to a plain object model with intrinsic relations modeled as
simple object attributes.

Heh. I gave that Jim Fulton quote, and encouraged the direction you describe; *and* I wrote zc.relationship. ;-) They are not mutually exclusive. Let's take a simple two-member directional relationship as an example--something like "A depends on B".

If neither object should inherently know about the relationship--it's not intrinsic to either of their data models--then it makes good sense to have external relationship objects that model the connection. The relationship container in zc.relationship is a simple example of a solution for that use case.

If the relationship is intrinsic to one object's data model but not to the other's, then it makes sense to have a Python pointer on the first object. If you need to ask questions, such as simple backpointers, then you can set up an extrinsic reference or you can set up a zc.relationship index to index the first object. For a simple backpointer case, an extrinsic reference is sufficient and simpler. If you need more sophisticated queries than just backpointers then the zc.relationship index can be a real win: you can adapt the first object to a relationship interface that breaks the relationship down into parts that match the queries you want to make. That works nicely.

If the relationship is intrinsic to both objects' data models, all you can really do is choose one of the previous two approaches and put some sugar around it. AIUI, schooltool's relationship code does something like this: you can specify both sides of a relationship in a class definition (and an interface, I believe) and behind the scenes it keeps track of the relationship for you in an external data structure, no matter which side of the relationship you modify. zc.relationship's index could be used to build functionality like this, but it has not been done to my knowledge.

The obvious question is: does this allow
powerful queries like the ones I mentioned in my first email or I should
switch and use extrinsic relations for this kind of features?

zc.relationship indexes can be combined with other catalog indexes to do what you described, but doing so efficiently will require knowledge of catalog indexes and the BTree module. The README for zc.relationship touches on this when it discusses filters:

| If relationship tokens (from 'findRelationshipChains' or 'apply' or
| 'findRelationshipTokenSet', or in a filter to most of the search methods) are | to be merged with other catalog results, relationship tokens should be based | on intids, as in the default. For instance, if some relationships are only | available to some users on the basis of security, and you keep an index of | this, then you will want to use a filter based on the relationship tokens
| viewable by the current user as kept by the catalog index.

The filter is not the only approach--you will sometimes want to merge *results* from a relationship index and other indexes, for instance-- but if you are doing transitive searches it may be preferred for efficiency.

zc.relationship is a bit of a powertool. Projects like the one Pete describes to make a friendly, more constrained usage of it will be very useful.

Also, don't forget SQL. I prefer the ZODB, and stay there if I can make a reasonably convincing case (almost always ;-) ), but sometimes an app really calls out for a hybrid approach.

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