[ZODB-Dev] Re: Setting the jar

[Florent Guillaume]

Tim Peters wrote:
> [Florent Guillaume]
>>>To be clear, I'm interested in how the _p_jar happens on a new object
>>that's just been created. Basically if you do:
>>
>># folder is a persistent object that has a _p_jar
>># folder.foo = Foo() # where Foo inherits from Persistent
>># here folder.foo doesn't have a _p_jar
>>transaction.commit(1)
>># here folder.foo has a _p_jar, how come ?
>>
>>I think I've missed something in my code trawling ;)
> 
> 
> Indeed, this will have you crawling on your belly for the next week <wink>.
> In effect, it all happens as a side effect of pickling, via the
> persistent_id() method of (in ZODB 3.4) class ZODB.serialize.ObjectWriter.
> The class docstring is telling the truth:
> 
>     """Serializes objects for storage in the database.
> 
>     The ObjectWriter creates object pickles in the ZODB format.  It
>     also detects new persistent objects reachable from the current
>     object.
>     """
> 
> but exactly how that happens requires tracing thru Python's C pickle code
> too.  Stick a print or a breakpoint here (inside persistent_id()) and you
> can watch it happening:
> 
>         if oid is None:
>             oid = obj._p_oid = self._jar.new_oid()
>             obj._p_jar = self._jar
>             self._stack.append(obj)
> 
> Note that ObjectWriter is passed a single object, but
> Connection._store_objects iterates over the ObjectWriter instance.  In your
> example, "folder.foo = Foo()" marks `folder` as changed, and the new object
> bound to .foo is discovered while pickling the new state for `folder`.
> Iterating over an ObjectWriter(obj) instance can end up discovering an
> arbitrarily large and complex graph of new objects reachable from obj, as it
> tries to pickle each in turn.  That's what the `self._stack` in the above is
> keeping track of.

Thanks, that's very helpful. I knew about pickling and persistent_id, but 
I'd never realized the one serializing for the storage had the side effect 
of adding the _p_jar to new objects.

Florent

-- 
Florent Guillaume, Nuxeo (Paris, France)   CTO, Director of R&D
+33 1 40 33 71 59   http://nuxeo.com   fg at nuxeo.com