如何在django-mptt中将子代的编号固定为父代？

MPTT_ADMIN_LEVEL_INDENT = 20

from django.contrib import admin
from mptt.admin import MPTTModelAdmin
from .models import YourModel

@admin.register(YourModel)
class YourModelAdmin(MPTTModelAdmin):
    list_display = ('indented_title',)

    def indented_title(self, obj):
        return '    ' * obj.level + str(obj)

    indented_title.short_description = 'Title'
    indented_title.allow_tags = True