Hi Milos, I'm not sure if I love it yet (I'm still playing) but this is a solution I've put into practice on one project that may work for you. I've defined two ZClasses, 'Relationship' and 'RelationshipItem'. I'm using ZPatterns, but I don't think that this affects the validity of the solution, it just allows me to switch to a RDBMS tomorrow if that turns out to be helpful. ;-) Relationship is really just a handy target. Relationship objects are created with unique ids, and they have a description property/field that can be used to describe particulars of any single relationship. RelationshipItems have several attributes: riType (this is the meta_type of the item involved in the relationship), originalID (the original id of the item in this relationship), relationshipID (the id of the relationship object) to which this relationshipItem is related, and finally relationshipRole (the role played by this item in this relationship). Now... in my Application I have 'mediaItems', 'Artists', and 'Locations'. An actual relationship consists of a set of zero or more items of these three types (clearly more types could be added without changing the system). A catalog is used (which would/could be replaced by SQL if that becomes a future direction.) To create a relationship between objects I (1) create a new Relationship object, and (2) create a RelationshipItem object for each item in the relationship. Here is an example method that gets Artists that have some relationship to a particular mediaItem: ## Script (Python) "getArtistIDsForMediaItem" ##bind container=container ##bind context=context ##bind namespace= ##bind script=script ##bind subpath=traverse_subpath ##parameters=mediaItemID ##title= ## theCatalog = container.Catalog theArtistIDs = [] theMediaItemBrains = theCatalog(riType='mediaItem', originalID=mediaItemID) for mib in theMediaItemBrains: theRelationID = mib.relationshipID artistBrains = theCatalog(riType='Artist', relationshipID=theRelationID) for ab in artistBrains: theArtistIDs.append(ab.originalID) return theArtistIDs It takes one catalog call to get all the Relationships for a particular mediaItem, and for each of those, I do one catalog call to find all the Artists in that relationship. With SQL you could do the whole lot on one call. With ZPatterns it's easy to use methods like this to add this method to the mediaItem object as an attribute called 'myArtistIDs' so that the programmer doesn't need to know *how* they are gotten, just that they are there. -steve On Monday, June 4, 2001, at 12:06 PM, Milos Prudek wrote:
I wonder if ZODB is suitable for the following problem, which I'll describe in relational terms:
There are articles and writers (authors). Each article has at least one writer, but it can have more than one writer. I need to list all articles of a given writer, and all writers of a given article.
A pure relation database solution would be as follows:
To escape putting list of writers in 'articles' table, there is an 'article-writer' table, called 'aw'. It contains valid combination of writers & articles.
aw table has these columns: a, w. articles table has these columns: a,other-data writers table has these columns: w,other-data
To list all articles of a writer No.1 : SELECT aw.a, articles.other-data FROM aw, articles, writers WHERE aw.w=1 AND writers.w=1 AND articles.a=aw.a
To list all writers of article No.2:
SELECT FROM aw,articles,writers WHERE aw.a=2 AND article.a=2 AND writers.w=aw.w
Is there an OO solution to this situation?
-- Milos Prudek, confused ZODB newbie
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