Bad relationships: RelationChoice, RelationCatalog and removed Dexterity content in Plone

Disclaimer: this post isn’t giving relationship advice ( is in some twisted way).  Unsurprisingly, removing content in Plone via the ZMI when Plone says something can’t be deleted is likely to cause problems.

In my case, the fact I removed a Dexterity-based container (“Project”) caused users to no longer add or edit content of my Project type.  The reason for this boiled down to my use of RelationChoice fields, in order link Projects to Person objects and vice versa.  The backend storage for the RelationChoice field is an instance of z3c.relationfield.index.RelationCatalog, and this keeps records of where the relations are going to and going from.

Now, I created a test Project and some content underneath it.  Due to some strange KSS errors I was getting on said content, I decided to try deleting the parent Project.  Plone complained, telling me that the content couldn’t be deleted, and giving the same KSS traceback errors. Needing to get rid of the Project, I decided to traipse into the ZMI and manually delete the Project that way.  I thought I was successful.  I was, but I caused more problems because the Project I made was still pointing at a Person object.

Several days later, users began complaining of not being able to add new Projects or edit existing Project content either.  The traceback looked like this:

2010-10-26 13:30:02 ERROR Zope.SiteErrorLog 1288063802.910.831336088181 http://localhost:8080/plone/programs/sample-project/@@edit
Traceback (innermost last):
  Module ZPublisher.Publish, line 119, in publish
  Module ZPublisher.mapply, line 88, in mapply
  Module ZPublisher.Publish, line 42, in call_object
  Module plone.z3cform.layout, line 64, in __call__
  Module plone.z3cform.layout, line 54, in update
  Module plone.dexterity.browser.edit, line 32, in update
  Module plone.z3cform.fieldsets.extensible, line 59, in update
  Module plone.z3cform.patch, line 30, in GroupForm_update
  Module, line 138, in update
  Module z3c.form.action, line 99, in execute
  Module z3c.form.button, line 311, in __call__
  Module z3c.form.button, line 170, in __call__
  Module plone.dexterity.browser.edit, line 21, in handleApply
  Module, line 117, in applyChanges
  Module zope.event, line 23, in notify
  Module zope.component.event, line 26, in dispatch
  Module zope.component._api, line 130, in subscribers
  Module zope.component.registry, line 290, in subscribers
  Module zope.interface.adapter, line 535, in subscribers
  Module zope.component.event, line 33, in objectEventNotify
  Module zope.component._api, line 130, in subscribers
  Module zope.component.registry, line 290, in subscribers
  Module zope.interface.adapter, line 535, in subscribers
  Module z3c.relationfield.event, line 82, in updateRelations
  Module zc.relation.catalog, line 546, in unindex
  Module zc.relation.catalog, line 556, in unindex_doc
  Module zc.relation.catalog, line 622, in _remove
KeyError: 128268893

These errors were strange, because the KeyError was different every time. After several hours of delving through code, it was clear that the ZC relation catalog was expecting a given key but not finding it (as the traceback suggests). After poking around in the RelationCatalog’s storage, there were a variety of references sticking around that involved the old Project that I’d deleted. I hacked in a:

print [x.from\_object for x in sorted(catalog.findRelations({}))]

at line 84 of z3c/relationfield/, and it showed references to objects that no longer existed.

Trying to use the removeRelations method of z3c/relationfield/ isn’t useful, because that swallows up the same KeyError, so in order to remove the borked relations, it would have to be done manually. That said, the underlying storage for the RelationCatalog goes over my head.

So, due to time constraints, I resolved my issues by using the catalog’s “clear” method to remove all records. Easily done by inserting:


at line 84 of z3c/relationfield/ (and removing after I edited a Project and invoked said code). Clearing the catalog didn’t break any of relations for my content, and everything’s now re-indexed after each Project was edited and saved again.

Word to the wise: if you’re working with this sort of tech, figure out why you can’t delete something and solve that, rather than hacking away. It will come back to bite you. Let me know if you’ve fixed a similar issue without needing to clear the catalog.

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