[Zope3-Users] Re: Object modified or not

[Philipp von Weitershausen]

Frank Burkhardt wrote:
> how does Zope decide, if a persistent object is modified?
> 
> What I'm interested in:
> 
> Does
>  
>    myobject._foo='bar'
>    delattr(myobject,'_foo')

del myobject._foo :)

> make Zope create a new version of the 'myobject' in zodb?

Yes. Persistent objects are regarded modified when

a) they have either received a new attribute,
b) or an old attribute has been set to a new value,
c) or an attribute has been deleted.

Attributes starting with _v_ are considered volatile and won't count in
this.

Note that a) through c) above specifically don't mention the changes
inside an attribute. Consider this:

  >>> some_persistent_obj.alist = []
  >>> transaction.commit()  # some_persistent_obj will be saved

  >>> some_persistent_obj.alist.append(1)
  >>> transaction.commit()  # nothing happens because
                            # some_persistent_obj itself
                            # hasn't changed

The workaround is to either make the 'alist' attribute a persisten
object of its own (e.g. a PersistentList). That would mean it gets its
own pickle and is stored independently of 'some_persistent_object'. This
is usually preferrable for large lists or dicts.

Another workaround is to explicitly mark 'some_persistent_obj' as changed:

  >>> some_persistent_obj.alist.append(2)
  >>> some_persistent_obj._p_changed = True
  >>> transaction.commit()


All of this is sort of explained in my book, but Gary Poster has
convinced me that it wasn't explained well enough. This will be fixed. :)

Philipp