I want a ManyToMany relationship between courses and departments - where the order of those mappings matters.

courses.py

class CoursePage(Page, ClusterableModel):
    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']