proposed bug fix for django-positions #26

bug_fixed_position_field.py

from positions import PositionField


class BugFixedPositionField(PositionField):

    def pre_save(self, model_instance, add):
        #NOTE: check if the node has been moved to another collection; if it has, delete it from the old collection.
        previous_instance = None
        collection_changed = False
        if not add and self.collection is not None:
            # Start bug fix
            try:
                previous_instance = type(model_instance)._default_manager.get(pk=model_instance.pk)
                for field_name in self.collection:
                    field = model_instance._meta.get_field(field_name)
                    current_field_value = getattr(model_instance, field.attname)
                    previous_field_value = getattr(previous_instance, field.attname)
                    if previous_field_value != current_field_value:
                        collection_changed = True
                        break
            except model_instance.DoesNotExist:
                pass
            # End bug fix
        if not collection_changed:
            previous_instance = None

        self._collection_changed = collection_changed
        if collection_changed:
            self.remove_from_collection(previous_instance)

        cache_name = self.get_cache_name()
        current, updated = getattr(model_instance, cache_name)

        if collection_changed:
            current = None

        if add:
            if updated is None:
                updated = current
            current = None
        elif updated is None:
            updated = -1

        # existing instance, position not modified; no cleanup required
        if current is not None and updated is None:
            return current

        collection_count = self.get_collection(model_instance).count()
        if current is None:
            max_position = collection_count
        else:
            max_position = collection_count - 1
        min_position = 0

        # new instance; appended; no cleanup required on post_save
        if add and (updated == -1 or updated >= max_position):
            setattr(model_instance, cache_name, (max_position, None))
            return max_position

        if max_position >= updated >= min_position:
            # positive position; valid index
            position = updated
        elif updated > max_position:
            # positive position; invalid index
            position = max_position
        elif abs(updated) <= (max_position + 1):
            # negative position; valid index

            # Add 1 to max_position to make this behave like a negative list index.
            # -1 means the last position, not the last position minus 1

            position = max_position + 1 + updated
        else:
            # negative position; invalid index
            position = min_position

        # instance inserted; cleanup required on post_save
        setattr(model_instance, cache_name, (current, position))
        return position