[Zope3-dev] substransactions and beforeDelete

[Florent Guillaume]

Jim Fulton wrote:
> Florent Guillaume wrote:
> 
>> I've been faced with the following Zope 2 problem in a consulting gig, 
>> and I'd like to be sure the use case is taken into account for Zope 3 
>> as I think it's quite important. I haven't been able to follow Zope 3 
>> lately, sorry for any duplication.
>>
>> We wanted to be able to have some objects veto their deletion in some 
>> cases (user has insufficient roles). This had to work even if it was a 
>> subobject of the object being deleted that had a veto, in which case 
>> the deletion of the toplevel object would also be vetoed.
>>
>> To do that in Zope 2, I first patched manage_beforeDelete to make it 
>> raise a subclass of BeforeDeleteException if the object vetoed 
>> deletion, and had the high-level code catch this exception to be able 
>> to display a message to the user explaining what happened and why 
>> deletion could not be successful.
>>
>> But, while before implementing it I'd always thought it would work, 
>> there is a problem. In the manage_beforeDelete recursion, other 
>> bookkeeping tasks are done, most notably removal from the CMF catalog. 
>> If we just raise an exception and catch it, the bookkeeping tasks for 
>> other objects walked before in the recursion will not be rolled back, 
>> even though no object is actually deleted... So actually doing the 
>> veto in the middle of the manage_beforeDelete recursion cannot work.
> 
> Why are you catching the exception?

Because I want to display a nice message to the user. If I don't catch 
the exception, it will go up to the Publisher that will display a 
not-nice error message. Hm are you suggesting I could provide a custom 
error message for that particular exception? That could work, but I 
would have like the flexibility of catching the exception from normal 
skin code and display my error message there.

>> What I would have liked to do is to be able to start a substransaction 
>> before the manage_beforeDelete thing is called, and either commit it 
>> or have it rolled back if my BeforeDeleteException is raised. This 
>> way, the unindexing would be rolled back. But from my understanding, 
>> Zope 2's ZODB cannot do that.
> 
> But it will be rolled back if an exception is raised.  I'm puzzled that
> you are catching the exception.
> 
>> So my question is, would this be possible in Zope 3 ? Setting a 
>> subtransaction point and doing a rollback of only that subtransaction ?
> 
> 
> well, of course, but this sounds too complex.
> 
>> (Note, to solve the problem in Zope 2, I now do a pre-deletion check 
>> that does a recursion and tests if deletion will be successful. If so, 
>> and only then, I do the real deletion.)
> 
> 
> You can roll-back subtransactions in Zope 2.  But the solution
> feels to complex to me.  I don't understand why you are catching
> and not reraising an error.
> 
> Jim
> 

-- 
Florent Guillaume, Nuxeo (Paris, France)
+33 1 40 33 79 87  http://nuxeo.com  mailto:fg at nuxeo.com