Plone 4: collective.formcriteria uninstall now can't import site - plone

Question

I had collective.formcriteria installed on a site, then it was uninstalled & removed from Buildout.
When I export my site it works fine, but trying to import the site gives me this error:
Traceback (innermost last):
Module ZPublisher.Publish, line 127, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 47, in call_object
Module OFS.ObjectManager, line 628, in manage_importObject
Module OFS.ObjectManager, line 646, in _importObjectFromFile
Module ZODB.ExportImport, line 92, in importFile
Module transaction._transaction, line 253, in savepoint
Module transaction._transaction, line 250, in savepoint
Module transaction._transaction, line 647, in __init__
Module ZODB.Connection, line 1128, in savepoint
Module ZODB.Connection, line 570, in _commit
Module ZODB.ExportImport, line 175, in _importDuringCommit
ImportError: No module named formcriteria.criteria.relativepath
Will installing this addon again and attempting to import/export my site work or is there another fix?
Thanks.

Answer 1

During import the ZODB has to remap the persistent ids in the dumped pickles, and to do so it has to 'load' all the entries in your export file.
During that remapping process it thus will try and import every referenced class. Your export still holds references to collective.formcriteria classes, and that's why you get the error during import.
There are two things you can do:
Install collective.formcriteria into your buildout just for this import to work. There is no need to run install code or anything, the Zope process just needs to be able to import the class during the import process. You can remove it again afterwards.
Fill in the missing imports with empty stubs. This is a bit more involved, as you'll need to create a dummy module that you'll insert into sys.modules to be loaded for this process.
The dummy module could look like this; I've named it 'placeholder.py':
from persistent import Persistent
class PlaceHolderClass(Peristent):
def __repr__(self):
return "<Persistent placeholder class for now-removed instances>"
And you stub out missing imports with:
import sys
import placeholder
sys.modules['collective.formcriteria.criteria.relativepath'] = placeholder
placeholder.FormRelativePathCriterion = placeholder.PlaceHolderClass
It is quite likely you'll need to stub out more such classes, based on the collective.formcriteria package layout.
Using this technique does not remove the broken references; it'll only let you import your .zexp file into a new instance.