Sorting pages with a parent_page_types restriction raises a MovePagePermissionError

Issue Summary

If you have a list of pages that are restricted to a certain parent page type and you toggle the sorting via “sort menu order”, sorting raises a MovePagePermissionError.

Steps to Reproduce

1. Start a new project with wagtail start myproject
2. Edit home/models.py, and add another page type:

class AnotherPage(Page):
    parent_page_types = ["home.HomePage"]

1. run python manage.py makemigrations and python manage.py migrate
2. Go to the Wagtail Admin, and add a few child pages under the Home page
3. While viewing the contents of the Home page, toggle the page sorting option via More -> Sort Menu Order:
4. Try to reorder the pages. The order is not being saved. My local development server outputs the following:

raise MovePagePermissionError(
wagtail.core.actions.move_page.MovePagePermissionError: You do not have permission to move the page to the target specified.

If I remove the parent_page_types restriction from the model definition, everything works as expected.

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: (yes / ~no~)

Technical details

• Python version: 3.9.7
• Django version: 3.2.12 and 4.0.2
• Wagtail version: 2.16.1
• Browser version: Firefox 96.0.3

[thijskramer]

Is there anything I have to do that prevents this PR from being merged?

[robmoorman]

Update: parent.specific_class is wrong (it’s the same as the cls in this case. But it’s definitely not the case.

The parent is wrong is this case. It’s not the parent but the target page (left/right pos)…

Making this parent (a trace higher) also wrong:

def can_move_to(self, parent):
    """
    Checks if this page instance can be moved to be a subpage of a parent
    page instance.
    """
    # Prevent pages from being moved to different language sections
    # The only page that can have multi-lingual children is the root page
    parent_is_root = parent.depth == 1
    if not parent_is_root and parent.locale_id != self.locale_id:
        return False

    return self.can_exist_under(parent)

This parent is also wrong.

Highest trace is this:

  def move(self, target, pos=None, user=None):
      """
      Extension to the treebeard 'move' method to ensure that url_path is updated,
      and to emit a 'pre_page_move' and 'post_page_move' signals.
      """
      return MovePageAction(self, target, pos=pos, user=user).execute()

This returns False in can_move_to:

        # reject moves that are forbidden by subpage_types / parent_page_types rules
        # (these rules apply to superusers too)
        if not self.page.specific.can_move_to(destination):

This code uses the can_move_to from a Page (see first code snippet here) and that’s expecint a truly parent but it’s not… as this is the destination page (left/rigth), so from the same page type.