[Zope] emulate a sequence

[Tino Wildenhain]

Jonathan schrieb:
> 
> ----- Original Message ----- From: "Yuri" <yurj at alfa.it>
> To: <zope at zope.org>
> Sent: Thursday, February 08, 2007 9:09 AM
> Subject: [Zope] emulate a sequence
>>
>> I need a field in zope object which act as a sequence. I mean every 
>> time I create an object, the value is the previous sequence value of 
>> the last object created + 1.
>>
>> also I need to browse the objects by sequence (from the object 1 to 
>> the last, by the sequence number)
>>
>> I think I need a catalog FieldIndex to store the sequence for 
>> searches, a property in the object.
>>
>> Can I use the catalog to retrieve the max value of the index, so when 
>> I create the object, I just add 1 and store it in the property?
> 
> You could use a catalog if you have other searching requirements, but 
> the method objectIds() (see the zopebook for more info) will return a 
> list of contained Ids, which you can then sort to get you what you 
> need.  Also, len(somefolder.objectIds()) will give you the next 
> available id (assuming your object ids have some sequential numbering).

Which both would not exacly emulate a sequence. Sequences live outside
transaction borders so they are inremented every time under any 
circumstances.

All the above methods would fall flat on the tummy when you are hit with 
variuos rollback or retries while other transactions concurrently add 
objects.

Regards
Tino