[Zope3-dev] Re: Recursive containment constraints

[Tres Seaver]

Stephan Richter wrote:
> On Wednesday 15 September 2004 09:01, Dominik Huber wrote:
> 
>>Month ago I read the contenttype part of stephan's book. at that time he
>>used
>>setTaggedValue to solve recursive containment constraints:
>>
>>    43  IMessage['__setitem__'].setTaggedValue('precondition',
>>    44
>>ItemTypePrecondition(IMessage))
>>    45  IMessage.setTaggedValue('__parent__', Field(
>>    46      constraint=ContainerTypesConstraint(IMessageBoard,
>>IMessage)))
>>
>>Today I wanted to use the setTaggedValue('__parent__', Field(...))
>>approach but it
>>seems not work anymore. Is that right?
> 
> 
> No, it should still work. I did not change to another pattern for technical 
> reasons.
> 
> 
>>    >>> '__parent__' in IMessage.names('all')
>>    False
>>
>>The new version of the book solves this problem with two indepentent
>>interfaces IMessage
>>and IMessageContained. Is that the 'official' pattern people should
>>follow?
> 
> 
> Yeah, I think this is the recommended pattern. I changed to this one, since it 
> is easier to read and understand. Also, all sample applications use this 
> pattern as well.

In reality, I think we would be better off moving the constraint 
declarations out of the classes;  that would both remove the "forward 
reference" problem and make it easy to move to a "containment constraint 
registry" solution when that became available.

My biggest reason for wanting this is that the constraints are *policy*, 
which might vary from site to site;  making them intrinsic to the class 
makes it harder to reuse the container / item classes in a site with 
different policies.  This consideration would suggest moving the 
constraint declarations to ZCML, e.g.:

   <content>
     ....
     <contains interface=".interfaces.IFooItem" />
   </content>
   <content>
     ....
     <container interface=".interfaces.IFooContainer" />
   </content>


Tres.
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Tres Seaver                                tseaver at zope.com
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