[OT] OODBs and schema, was Re: [linux-audio-user] Re: sound & midi applications page, was Re: [Consortium] linuxaudio.org

[Paul Winkler]

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)