Django admin，ManyToManyField>外键关系

Question

假设我有这些模型（显然省略了一些属性）：

class Product(Model):
    name = CharField(max_length=100)

class Variety(Model):
    product = ForeignKey(Product)

class ProductMerge(Model):
    parent_product = ForeignKey(Product)
    products = ManyToManyField(Product, related_name='merge_proposals')

如何在管理面板的内联中显示所有Product的ProductMerge变体

我尝试在TabularInline上设置自定义get_queryset：

def get_queryset(self, request):
    return Variety.objects.filter(
        product__in=self.instance.parent.child_products.all())

但我有以下例外：

ValueError: 'marketplace.Variety' has no ForeignKey to 'marketplace.ProductMerge'.