I want a ManyToMany relationship between courses and departments - where the order of those mappings matters. The courses.py models work just fine in the admin UI but we are struggling with creating a script that updates the mappings and still leaves the pages editable in the Wagtail admin.

course_changes_importer.py

class CourseChangesImporter(object):
  
    def sync_with_cataloger(self, catalog_label, json_data, course_listing_page, dry_run=False):
        courses = json_data.get('courses', [])
        for course in courses:
            proposal_type = course['proposal_type']
            course_number, course_letters = self._split_course_number(course['course_number'])
            if proposal_type == "CHANGE":
                course_page = self._find_current_course_page(course_listing_page, course['copied_from'], dry_run)
                course_page = course_page.get_latest_revision_as_object()

                if course_page:
                    self._set_course_page(course_page, course, dry_run)
                    if not dry_run:
                        course_page.save()
                        course_page.save_revision(log_action=True).publish()
                else:
                    course_label = self._get_course_label(course['orig_departments'], course['orig_course_number'])
                    logger.error(
                        f"{proposal_type} {course_label} {course['course_title']} FAILED",
                        proposal_id=course['proposal_id'],
                        copied_from=course['copied_from'],
                    )


    def _set_course_page(self, course_page, course_data, dry_run=False):
        """
        Assigns JSON data from cataloger to CoursePage.
        """
        course_number, course_letters = self._split_course_number(course_data['course_number'])
        course_page.course_number = course_number
        course_page.course_letters = course_letters
        course_page.description = (course_data['course_description'] or '').strip()
        
        # If departments have changed, recreate the coursedepartment mappings
        if not dry_run and (not course_data['orig_departments'] == course_data['departments']):
            imported_depts = []
            for idx, dept_dict in enumerate(course_data['departments']):
                dept = Department.objects.get(exeter_department_id=dept_dict['exeter_department_id'])
                imported_depts.append(CourseDepartment(course=course_page, department=dept, sort_order=idx))
            course_page.coursedepartments.set(imported_depts)

courses.py

class CoursePage(Page):
    course_number = models.CharField(
        verbose_name='Course Number',
        max_length=256,
        help_text="Only the numeric part",
    )
    course_letters = models.CharField(
        verbose_name='Course Letters',
        max_length=256,
        blank=True,
        help_text="Only the letters (aka 'abc')",
    )
    description = RichTextField(
        verbose_name='Course Description',
        blank=True,
        editor='course_description',
    )
    
    departments = ParentalManyToManyField('core.Department', related_name="courses", through='core.CourseDepartment')

    
    content_panels = [
        FieldPanel('title'),
        InlinePanel('coursedepartments', label="Departments", min_num=1),
        FieldPanel('course_number'),
        FieldPanel('course_letters'),
        FieldPanel('slug'),
        FieldPanel('description'),
        CommentPanel(),
    ]

    # CNK we don't need the promote or publish tabs so combine everything into one content tab
    edit_handler = TabbedInterface([
        ObjectList(content_panels, heading='Content'),
    ])
    
    
class CourseDepartment(Orderable):
    course = ParentalKey('core.CoursePage', related_name='coursedepartments', on_delete=models.CASCADE)
    department = models.ForeignKey('core.Department', related_name='coursedepartments', on_delete=models.CASCADE)

    panels = [
        FieldPanel('course'),
        FieldPanel('department'),
    ]

    class Meta:
        # The ordering will affect the API
        ordering = ['sort_order']


@register_snippet
class Department(RevisionMixin, models.Model):
    """
    Model representing a Department for the Course Catalog.
    """

    abbr = models.CharField(
        verbose_name='Department Abbreviation',
        max_length=100
    )
    name = models.CharField(
        verbose_name='Department Name',
        max_length=100
    )
    is_active = models.BooleanField(
        verbose_name='Is Active',
        default=False,
        help_text="True if department is used for the current catalog.",
    )

    class Meta:
        verbose_name = 'Department'
        ordering = ['name']