def _migrate_abstract_field_values(self, old_abstract):
        fields = dict(getattr(old_abstract, '_fields', {}))
        fields.pop('content', None)
        for field_id, field_content in fields.iteritems():
            value = convert_to_unicode(getattr(field_content, 'value', field_content))
            if not value:
                continue
            try:
                new_field = self.legacy_contribution_field_map[field_id]
            except KeyError:
                self.importer.print_warning(cformat('%{yellow!}Contribution field "{}" does not exist')
                                            .format(field_id),
                                            event_id=self.event.id)
                continue
            new_value = self._process_contribution_field_value(field_id, value, new_field, AbstractFieldValue)
            if new_value:
                if not self.importer.quiet:
                    self.importer.print_info(cformat('%{green} - [field]%{reset} {}: {}').format(new_field.title,
                                                                                                 new_value.data))
                yield new_value