[Zope] one 'document' in many 'folders'

[Jim Fulton]

Stefan Franke wrote:
> 
> Jim Fulton wrote:
>         > Sorry you didn't get an answer.  There is no problem with
> creating
>         > acyclic or cyclic graph constructors in the, uh, object
> hierarchy,
>         > which BTW, need not be a hierarchy. :)
> 
> Not too long ago I experimented a little with cyclic structures
> inside Zope's database. One thing I noticed was, that the copy
> support failed with tracebacks like this:
> 
> Traceback (innermost last):
>   File D:\Programme\zope\koelnzope\lib\python\ZPublisher\Publish.py,
> line 877, in publish_module
>   File D:\Programme\zope\koelnzope\lib\python\ZPublisher\Publish.py,
> line 590, in publish
>     (Info: /BLS/1000/ADV-Angelegenheiten/manage_pasteObjects)
>   File D:\Programme\zope\koelnzope\lib\python\OFS\CopySupport.py, line
> 176, in manage_pasteObjects
>     (Object: Navigation)
>   File D:\Programme\zope\koelnzope\lib\python\OFS\CopySupport.py, line
> 361, in _getCopy
>     (Object: Navigation)
>   File D:\Programme\zope\koelnzope\lib\python\BoboPOS\PickleJar.py, line
> 327, in export_file
>   File D:\Programme\zope\koelnzope\lib\python\BoboPOS\SimpleDB.py, line
> 958, in exportoid
> ExportError: Object 184337 references itself
> 
> This led me to the conclusion to better leave my hands
> off non-hierarchic data structures which differ from those
> created by the management interface itself.

I'll admit that cyclic data structures are pretty rare, 
so handling them may not be totally correct in every case.

Export of objects partcipating in cycles is potentially, 
uh, confusing, unless the cycles are with objects that are totally
contained within the object being exported, although containment
is pretty hard to check.  I should probably take this error check out.

>         >> Would also like to hear more about this...
>         > Like what? :)
> 
> More or less simple answers like "this is ok and this
> is not",

OK, but I need to know what "this" is.  I don't think
you want me to enumerate all possible "this"s. ;)

> or "you can store any complex Python data
> structure inside the DB,

You can store any complex Python data in the DB subject to:

  - It must be picklable,

  - It must follow the persistence rules:
 
    - Root objects must be "persistent" meaning
      they subclass BoboPOS.Persistent (or implement
      an undocumented interface that BoboPOS.Persistent
      implements, sigh)

    - Subobjects of persistent objects must be "persistent" or
      they must be immutable, or be used immutably, as in:

         def add_member(self, member):
	      members=self.members

              # Oops, mutated a list
              members.append(member)

	      # OK, so pretend it's new and readd it
              self.members=members
         

> but in conjunction with the
> management interface, you get the following drawbacks.."

Basically, the objects need to implement certain
interfaces if they want to play in the management system.
If they don't want to play, they have alot of freedom.

We need to do a much better job of documenting these interfaces.
 
> I'm still a quite unclear about the object management
> in the database itself

The object management at the database level tries to be as 
transparent as possible.  The wish is that you don't have to
think about it.  Basically, if you follow the rules above, 
you should be OK.  

Probably the biggest pitfall in the database is the inability
to get to database objects when their classes can no longer be 
imported, because classes or modules have been moved or deleted
or because errors have been introducted into the module source.
We need to figure out ways to make these problems easier to
diagnose.

> and the management layer Zope adds.
> But maybe I just have to read the docs harder ;-)

Maybe you just need to read my mind better. ;)

Jim

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