Pickling Error with RelationList inside DictRow in AddForms

[adrianschulz]

After the update to Plone 5.2 with Python3, we noticed a pickling error in the AddForms of some contenttypes. After further investigation, we noticed that all affected contenttypes have a behavior with a contacts-field which is a DataGridField with a RelationList (see the following code).

class IContactRoleSpecification(Interface):

    role = schema.Choice(vocabulary=RoleVocabulary,
                         title=_(u'label_Role', default=u'Role'),
                         required=True)

    widget('contact',
           CustomRelatedItemsWidget,
           location_folders={'de': '/Plone/de/personen',
                             'en': '/Plone/en/people'},
           additional_favorites=[{'title': _(u'Current Context'),
                                  'de': '{Context}',
                                  'en': '{Context}'},
                                 {'title': _(u'Start Page'),
                                  'de': '{Start}',
                                  'en': '{Start}'},
                                 {'title': _(u'Contacts-Folder'),
                                  'de': '/Plone/de/personen',
                                  'en': '/Plone/en/people'}],
           selectableTypes=['Kontaktperson'],
           overrideRootPath=True,
           )
    contact = RelationList(title=_(u'label_Contacts', default=u'Contacts'),
                           value_type=RelationChoice(title=u'Contact',
                                                     source=CatalogSource()),
                           required=False,
                           missing_value=[],
                           default=[])


@provider(IFormFieldProvider)
class IDynamicContacts(model.Schema):

    directives.widget(contacts='collective.z3cform.datagridfield.DataGridFieldFactory')
    contacts = schema.List(
        value_type=DictRow(title=_(u'Contact with Role'),
                           schema=IContactRoleSpecification),
        title=_(u'label_contact_with_role', default=u'Contact with Role'),
        description=_(u'help_contact_with_role', default=u''),
        required=False)

If we save the object with an empty contacts-field we don’t get the pickling error. If we fill in a value the following error gets thrown.

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 156, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 338, in publish_module
  Module ZPublisher.WSGIPublisher, line 256, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 62, in call_object
  Module plone.z3cform.layout, line 63, in __call__
  Module plone.z3cform.layout, line 47, in update
  Module plone.dexterity.browser.add, line 141, in update
  Module plone.z3cform.fieldsets.extensible, line 65, in update
  Module plone.z3cform.patch, line 30, in GroupForm_update
  Module z3c.form.group, line 145, in update
  Module plone.app.z3cform.csrf, line 22, in execute
  Module z3c.form.action, line 98, in execute
  Module z3c.form.button, line 315, in __call__
  Module z3c.form.button, line 170, in __call__
  Module plone.dexterity.browser.add, line 116, in handleAdd
  Module z3c.form.form, line 265, in createAndAdd
  Module plone.dexterity.browser.add, line 91, in add
  Module plone.dexterity.utils, line 189, in addContentToContainer
  Module Products.BTreeFolder2.BTreeFolder2, line 464, in _setObject
  Module zope.event, line 32, in notify
  Module zope.component.event, line 27, in dispatch
  Module zope.component._api, line 124, in subscribers
  Module zope.interface.registry, line 442, in subscribers
  Module zope.interface.adapter, line 607, in subscribers
  Module zope.component.event, line 36, in objectEventNotify
  Module zope.component._api, line 124, in subscribers
  Module zope.interface.registry, line 442, in subscribers
  Module zope.interface.adapter, line 607, in subscribers
  Module plone.app.versioningbehavior.subscribers, line 109, in create_initial_version_after_adding
  Module Products.CMFEditions.CopyModifyMergeRepositoryTool, line 336, in save
  Module Products.CMFEditions.CopyModifyMergeRepositoryTool, line 500, in _recursiveSave
  Module Products.CMFEditions.ArchivistTool, line 270, in prepare
  Module Products.CMFEditions.ArchivistTool, line 224, in _cloneByPickle
TypeError: Can't pickle objects in acquisition wrappers.

Again we investigated further and found the referenced object in the contacts-field is not a RelationValue and also wrapped in Acquisition-Wrappers (as the error-message shows).

Shouldn’t the value be stored as RelationValue? If not: Why?
Does anybody know why we encounter the problem only when adding an object and not when editing it?

As a workaround, we are now hiding the field in all the AddForms.

Relevant versions:

collective.z3cform.datagridfield = 1.5.2
plone.app.relationfield = 2.0.1
z3c.relationfield = 0.9.0
zc.relation = 1.1.post2
z3c.form = 3.7.0
plone.app.z3cform = 3.1.3
plone.autoform = 1.8.1

NOTE: For the described field we have a custom Serializer, Deserializer, and RelatedItemsWidget. But our tests without these custom elements showed the same error.