[Zope3-Users] Re: folder list in a vocabulary

[Michel Pelletier]

Stephan Richter wrote:

> 
> Note that I would strongly suggest using a relationship framework for this 
> sort of use case. A good API is provided by ``schooltool.relationship`` and 
> is heavily used in the SchoolTool code, so you have many examples.

I've spent some time recently studying schooltool.relationship and it is 
simple and sufficient if you have a small number of relationships.  I 
like the Pythonic nature of its interface and it strongly leverages Zope 
3 adaptation.  However, if you need to search for either the referer, 
the reference, or the referent, it goes through 
relationship.LinkSet.find(), which is implemented as:

def find(self, my_role, target, role, rel_type):
         for link in self:
             if (link.rel_type == rel_type and link.target is target
                 and link.my_role == my_role and link.role == role):
                 return link
         raise ValueError(my_role, target, role, rel_type)

This is, worst case, linear search time as your relationship set grows. 
  Note also that relationship.relate() is guaranteed linear time (it 
must  check for duplicates) and unrelate() is worst case linear time.

One could, of course, catalog the relationships and re-implement find(), 
relate() and unrelate() to be much more efficient.  One of the beauties 
that components gives us!

For a similar relationship library that uses a faster indexing method, 
and supports multiple relationship store backends (ZODB, sleepycat, 
SQLObject, redland, cwm, 4Thought), supports W3C standard relationship 
interchange formats (RDF/XML, N3), and comes with a standard SQL -like 
query language (Sparql), see http://rdflib.net.  There is a Zope 3 
product called Zemantic at codespeak which defines the necessary ZCML to 
register rdflib classes with Zope 3.

rdflib could be way too much for you, and schooltool.relationship just 
right.  rdflib development was started before Zope 3 existed, so it is 
not as component oriented, but this is not really a disadvantage, the 
vast majority of existing Python libraries are in this same boat.  It 
depends on your use cases.  If I never had more than a thousand 
relations (in your case a few hundred book, a few dozen authors), I'd 
seriously consider using schooltool.relationship.  We've tested rdflib 
with over 2 million relations on modest hardware.

-Michel (@cignex.com)