Drag and drop sorting with polymorphic models without any nesting always sets beyondMaxLevels too large

[cdroege]

I have the following example code for the admin interface:

class ClassInline(nested_admin.NestedStackedPolymorphicInline):
    class SomeClassInline(nested_admin.NestedStackedPolymorphicInline.Child):
       model = SomeClass
    # [...] other elements of NestedStackedPolymorphicInline.Child
    model = BaseClass
    child_inlines = [
       SomeClassInline #, [...]
    ]
    sortable_field_name = 'order'


class ClassLevelAdmin(nested_admin.NestedPolymorphicModelAdmin, admin.ModelAdmin):
    model = ClassLevel
    inlines = [
       ClassInline,
    ]
    list_display = [
       'name',
    ]


admin.site.register(ClassLevel, ClassLevelAdmin)

When I want to sort inlines via drag and drop in the admin interface the following happens:

1. The background of the inline has a red background and the element in the DOM has the error class "ui-nestedSortable-error"
2. This happens immediately when clicking on the inline without moving it between other inlines
3. Moving the inline between other inlines does not work and it will be moved back to the starting position
4. The this.beyondMaxLevels variable (in the Javascript code) is set to 1. This should probably be always 0 in this case as there is only a flat hierarchy of inlines and my guess is a problem with the calculation of the levels

Another admin interface for another model with a nested polymorphic inline works:

class ClassInline(nested_admin.NestedStackedPolymorphicInline):
    class SomeClassInline(nested_admin.NestedStackedPolymorphicInline.Child):
       model = SomeClass
       inlines = [SomeNestedInline, ]

It would be nice if the sorting feature of django-nested-admin could work without any nested inlines as it also supports django-polymorphic. Thank you for your module!

• Django 2.2.13
• Django Nested Admin 3.3.1
• Firefox and Chrome used in the admin interface

[fdintino]

I've tried several different variations on your example code and am unable to reproduce this. Could you provide a test repo (or even just a functional models.py and admin.py) that I could use to troubleshoot this?