[Zope-dev] relations in objects

[Tres Seaver]

On Fri, 2003-10-10 at 05:51, Bjorn Stabell wrote:

> > From: Paul Winkler [mailto:pw_lists at slinkp.com] 
> >> On Thu, Oct 09, 2003 at 08:46:24PM -0700, Jason Corbett wrote:
> >> How does an object in zope know where it sits in the hirearchy,
> > 
> > The only unique identifier that zope provides is
> > the containment path. An object can tell you its containment 
> > path by doing foo.absolute_url(1) or foo.getPhysicalPath() 
> > depending on what form you  want the output in.
> 
> I am often confused by this as well.  I think, however, that you can
> also reach objects by its "object id" oid.  If I just use simple
> assignment of a variable to an object in Zope, I get that object's oid,
> right?

Yes, but only at the ZODB level, and only if the "contained" object
derives from Persistent.  From the application's point of view, OIDs are
irrelevant and useless:  there is no API for manipulating them sanely.

For example:

  from OFS.SimpleItem import SimpleItem # derives from Persistent

  class Foo( SimpleItem ):
      pass

  owning = Foo()

  owned = Foo()

  owning.owned = owned

While it is true that the ZODB will pickle only the OID of 'owned'
within the dict of 'owning', the application doesn't need or want to
manipulate the OID, because it can just refer to 'owning.owned'.

This kind of reference is conceptually like a "hard link" in a
traditional Unix filesystem;  the same "file" can be stored in multiple
"directories", because the directory contains only a mapping of filename
to "inode" (OID).

The analog for a "symlink" would be to store a path to the object, and
then use '{,un}restrictedTraverse' to fetch the object when needed. 
There are a number of products available which try to make such
"symlinks" more transparent to the application;  it is, however, harded
to make this work cleanly in the general case.

Tres.
-- 
===============================================================
Tres Seaver                                tseaver at zope.com
Zope Corporation      "Zope Dealers"       http://www.zope.com