The discussion on Gemstone isn't the first time that people seem to
think the ZODB doesn't do what it actually does. If you have object A
that points to B, and object C that also points to B, updating B will
update B truly, and A and C will both aware of this change.
Quite a few smart people seem to be under the impression the ZODB
doesn't actually take care of this. They believe that B will be
persisted twice, once for A, once for C. This may be the case if B
doesn't inherit from Persistent, but if it does, there really will only
be that instance.
The ZODB is quite transparent that way. It's almost exactly like a pool
(typically all stored in the same root dictionary) of Python objects.
They can reference each other just fine. The only requirements I know of
* if you don't inherit your class from Persistent, or use a python
builtin (which doesn't inherit from Persistent), things will be
serialized multiple time, as far as I'm aware. (I may be wrong)
* if you have a non-Persistent subobject (like a list) and you change
it, you need to manually flag the persistence machinery on the object
that its subobject changed, with _p_changed. This is *only* necessary if
some of the objects are not Persistent. For common built-in collections
in Python such as list and dictionary there are replacements
(PersistentList, PersistentMapping), and more advanced building blocks
for indexes (BTrees), that don't have this issue.
Anyway, the misapprehension that the ZODB somehow does less than it does
seems to be an easy one for people to develop. I think it would be
important to show on the ZODB home page that this is truly not the case.
This needs to be repeated early and often.
The *reason* people don't use hard references like this all the time in
an application is that sometimes you want back references, and sometimes
you want loose coupling. So that's when things are referenced by a
string or a lookup or whatnot. Just like you sometimes put Python
objects in a dictionary with keys. The wide application of such soft
references seems to give people the impression you can't point the same
object from tree X and Y at the same time.
For more information about ZODB, see the ZODB Wiki:
ZODB-Dev mailing list - ZODB-Dev@zope.org