Best approach to handle broken relations in Dexterity

fredvd · February 12, 2016, 11:05am

Hi,

This is a known problem in plone.app.relationfield. I created an issue for it here (in 2014)

github.com/plone/plone.app.relationfield

p.a.relationfield doesn't clean up relations to removed content for Dexterity CT's

opened 10:37AM - 25 Sep 14 UTC

closed 06:07AM - 30 Nov 22 UTC

fredvd

01 type: bug 12 prio: high 21 status: confirmed

Related items using plone.app.relationfield with Dexterity Items are broken at t…he moment when you delete related contents. This manifests itself in for example plone.app.contenttypes implementations, both in Plone 4.3.x with p.a.c 1.1bX as well as in Plone 5 coredev. Specifically what is broken is that if you create a related item from contentitem 1 to content item 2 and then remove content item 2, the relation is not cleaned up and iterating over the relations in content item 1 breaks. We stumbled upon this when creating a custom content type with relations using plone.app.relationfield. The problem doesn't occur with Archetypes based relations. There are event subscriptions in z3c.relationfield for IObjectRemovedEvent and IObjectModifiedEvent to clean up relations and they work for Archetypes CT's (like using the default related items functionality in Plone 4.3.X) but not with Dexterity CT's as in p.a.contenttypes. The quick fix for my custom CT is to iterate over all relations and to filter those out with r.isBroken() : [r for r in self.context.related_articles if not r.isBroken()] and I use those in views. --- Feedback From Philip Bauer on the Plone developer Mailinglist: from a first glimse at the issue it seems plone.app.relationfield should make sure that the objects with relations implement z3c.relationfield.interfaces.IHasOutgoingRelations or the event-handler in z3c.relationfield also handles plone.app.relationfield.interfaces.IDexterityHasRelations. The main problem is that plone.app.relationfield does not have the relevant tests. (#2)

In archetypes days, when you delete content, p.a.relationfield (or actually one of the 'lower' packages in z3c.relationfield were listening in on events and cleaned up the incoming references on other object to X if object X was removed. This is broken and doesn't work anymore. See the ticket for more info, I figured it out half way back then but it got too complex for me.

Unless there have been fixes down in the meantime or other solutions, the default related items viewlet in plone 5 will break as well if you remove objects you relate to.

The 'fix' I'm doing in custom projects is filtering the relations using a list comprehension and isBroken:

def existing_new_articles(self):
    """ filter the relation list to only have relations to still existing
        content """
    articles = []
    if hasattr(self.context, 'new_articles'):
        articles = [a for a in self.context.new_articles
                    if not a.isBroken()]