Subject: [OT] OODBs and schema, was Re: [linux-audio-user] Re: sound & midi applications page, was Re: [Consortium] linuxaudio.org
From: Paul Winkler (pw_lists_AT_slinkp.com)
Date: Mon Feb 02 2004 - 18:15:43 EET
On Mon, Feb 02, 2004 at 02:55:02PM +0000, Steve Harris wrote:
> On Mon, Feb 02, 2004 at 09:36:02 -0500, Paul Winkler wrote:
> > > and wether you are willing to change.
> >
> > I do web development with Zope. My site code never depends on
> > the data store :-)
>
> I'm not sure if thats a joke or not ;) Zope is based on an OODB IIRC.
> thier not very good at schema independence in my experience.
I'm not sure what "schema independence" means, so I will
assume it means the ability to extend or modify the
schema after you've already got live instances.
I'm curious what languages you've used with an OODB.
I imagine it would be a major pain if you didn't have as much
runtime flexibility as python (e.g. ruby would be just fine,
C would be insane, dunno about java).
It's been a non-issue in the 3 years I've been doing Zope.
Content types in zope are generally defined with classes.
Adding an attribute to a class requires me to add a
couple of lines to the class definition, including
one to provide a default class-level value, then I tell zope
to reload the class by clicking a button in the zope management
interface. Live instances immediately see the default
value. This requires no deep magic, it's just how python works.
An untested-but-should-work example (feel free to skip
the rest of this message):
from ZODB import Persistence
class SoftwareResource(Persistence.Persistent):
meta_type='Software Resource'
def __init__(self, name='', author='', download_url=''):
self.name=name
self.author=author
def setAuthor(self, author):
# setter methods are totally optional in python,
# but we usually use them in zope so we can
# declare security restrictions on them,
# which I won't bother with in this example.
self.author = author
def setName(self, name):
self.name = name
Now let's add a release_number attribute with a default
value of '':
class SoftwareResource(Persistence.Persistent):
release_number = ''
...
def __init__(self, name='', author='', release_number=''):
...
self.release_number=release_number
...
def setRelease(self, release_number):
self.release_number=release_number
Reload the class and you're done.
Changing the 'author' attribute to 'author_name'
is only barely more difficult. You can embed self-upgrading
code in the class, but such code has a way of sticking around
long after it should be deleted. Another way to do it is with a
trivial upgrade script like this, to be run once after
reloading the relevant class changes:
# A bit like find . -type ...
for obj in context.ZopeFind(meta_type='Software Resource'):
if not hasattr(obj, 'author_name'):
obj.setAuthor(obj.author)
del(obj.author)
--Paul Winkler http://www.slinkp.com Look! Up in the sky! It's THE FISHY ANUS! (random hero from isometric.spaceninja.com)
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