Plone's in-place conversion of File (ATBlob) to Image (ATBlob)

convert.py

# -*- coding:utf-8 -*-
from plone.app.blob.interfaces import IATBlobFile
from plone.app.blob.migrations import ATFileToBlobMigrator
from plone.app.blob.migrations import migrate
from Products.contentmigration.walker import CustomQueryWalker
from Testing.makerequest import makerequest
from zope.component.hooks import setSite
from zope.interface import noLongerProvides

import transaction

app = makerequest(app)
site = app.ieausp
setSite(site)


class BlobFileToBlobImageMigrator(ATFileToBlobMigrator):
    ''' Migrate File (ATBlob) to Image (ATBlob) '''

    src_portal_type = 'File'
    src_meta_type = 'ATBlob'
    dst_portal_type = 'Image'
    dst_meta_type = 'ATBlob'
    # migrate all fields except 'file', which will be moved to 'image'
    fields_map = {'file': None}

    def migrate_data(self):
        value = self.old.getField('file').getAccessor(self.old)()
        self.new.getField('image').getMutator(self.new)(value)

    def finalize(self):
        ATFileToBlobMigrator.finalize(self)
        # Remove IATBlobFile interface, otherwise schemaextender yells
        # about primary field already set
        noLongerProvides(self.new, IATBlobFile)


additionalQuery = {'path': '/Plone/images/'}

walker = CustomQueryWalker(site, BlobFileToBlobImageMigrator,
                           use_savepoint=False,
                           query=additionalQuery,
                           src_portal_type='File',
                           dst_portal_type='Image')


try:
    migrate(site, walker())
except:
    # If something goes wrong, let's find out
    import pdb;pdb.post_morten()

# Commit the whole migration
transaction.commit()

# Sync zeo
app._p_jar.sync()

Code doesn't work for me. I still get an exception about setting another primary field.