如何在Django的ManyToManyField关系中过滤模型相关的类？

Question

我想过滤与模型House 相关的类Location。这个想法是根据管理面板中字段country 的值来限制Location 的选择。有可能吗？如果可以，我该怎么做？

class House(models.Model):
    country = models.ForeignKey(Country,
                                 on_delete=models.CASCADE,
                                 related_name='country'
                                 )
    city = models.ManyToManyField(
        Location,
        related_name='city'
    )

基本上我想要这样的东西，但是 Django 抛出一个错误，说 django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet。

city = models.ManyToManyField(
    Location.objects.filter(country=country,
    related_name='city'
)

Answer 1

class HouseAdmin(admin.ModelAdmin):
    def formfield_for_manytomany(self, db_field, request, **kwargs):
        if db_field.name == "city":
             kwargs["queryset"] = Location.objects.filter(country=country)
        return super(HouseAdmin, self).formfield_for_manytomany(db_field, request, **kwargs)

来源Django filter many to many field in admin?