On May 13, 2007, at 12:39 PM, David Binger wrote:

>
> On May 13, 2007, at 12:53 PM, David K. Hess wrote:
>
>> Is it possible to use a Persistent object as a key in a BTree? I'm
>> having difficulties and suspect the problem has to do with
>> Python's default object comparison behavior since Persistent
>> doesn't appear to implement __cmp__. Since Python uses a (dynamic)
>> arbitrary ordering of in-memory objects for default cmp() behavior
>> it seems to me that BTree will not work right. I.e. it needs a
>> persistent ordering of the objects.
>
> You are correct that Persistent objects won't work correctly in a
> BTree unless you
> do something yourself to make sure that the ordering never changes
> from one
> process to another.
>
>>
>> If that is the root of the problem, any thoughts on how best to
>> make this work? Using OID for comparison is the obvious first
>> choice except there isn't one until after the first commit of the
>> object. Should I add a serial number to my Persistent subclass and
>> implement __cmp__ as a comparison of it?
>
> I don't think applications should depend on oid values for anything.
> I think it is important to preserve the potential to
> re-assign OIDs without mangling your application data.
> A serial number, name, or some other application-level identifier
> would probably work.
>
> As keys in a BTree, Persistent instances
> have a disadvantage over non-persistent values in that the
> persistent state of the keys will need to be retrieved from the
> database, if it is not already loaded, for every key that is compared
> to a key that you are trying to find or insert.   If there
> is a reasonable alternative for keys, be sure to
> consider it.

Oh, good point. I think a change from an object reference to the
actual id I'll be using will work in my case.

Thanks!

Dave

------
David K. Hess
Verscend Technologies, Inc.
dhess@verscend.com
214-684-5448