[Zope-dev] Unique and Contant object IDs

[Ty Sarna]

In article <000701c03d28$63f28c00$ac0aa8c0@tor.torped.se>,
Johan Carlsson <johanc@torped.se> wrote:
> Hi Zen-guys,
> 
> Is there an unique object ID in the ZODB that remains constant in all times
> through out the life time of the ZODB?
> And if so, where is it created and how do I access it? In other word how do I use it?
> 
> I can vaguely recall seeing something called OID.

You can (from python) access the _p_oid attribute of an object to get
it's OID, but not all that useful. Given an oid, there's no way to get
back to a useful object. I've thought about patching _setObject to store
a "hard" parent pointer, thus making it possible to walk back up the
tree and figure out how to create a properly wrapped version of the
object as it it had been accessed via it's canonical path. There are
complications like mountable storages though.

Unfortunately there are a lot of things that Zope just can't do because
there is no way to get a persistent "ticket" for an object that can be
handed out to some external system, and then later redeemed for the
(properly wrapped) object. Pathnames are not useful, because they don't
last for the object's lifetime.

I haven't tried it, but I'm almost positive the semantics of ZServer's FTP
are a bit wonky because of this. For some other things I'd like to do,
it's a fatal blow.

> I need it to track discussions related to an object, but stored in a
> SQL-database. I been pondering a traversal interface in the "commentable"
> objects, that would give the illusion that the discussion items really are object
> agregated in the "commentable".
> 
> I also guess this could be something ZPatterns could solve in the future?

For this type of use, ZPatterns can easily solve the problem.

One way to do this: Make your objects DataSkins-derived, and set up
SkinScript rules so that when the sql_key property is accessed, a new
key is generated and the object's sql_key is set to it.  Now, anytime
you ask for object.sql_key, you either get one it was assigned
previously (the SkinScript attribute providers are only used if a
persistent attribute can't be found) or a new one is generated and
stored in the (persistent) object for future use. 

Now you can write other SkinScript that keys off of self.sql_key to
provide access and storage of whatever other attributes from/to SQL.