如何仅获取模型的关系字段？

Question

这是我的模型，问题如下。

from django.db import models

# ...

class County(models.Model):
    name = models.CharField(max_length=50, verbose_name=_("Numele județului"))
    capital = models.ForeignKey("Municipality", on_delete=models.PROTECT, null=True, blank=True, related_name="capital_of", verbose_name=_("Reședința"))
    region = models.ManyToManyField(Region, related_name="counties", verbose_name=_("Regiunea"))
    area = models.IntegerField(verbose_name=_("Suprafața"))
    population = models.IntegerField(verbose_name=_("Populația"))
    description = models.TextField(verbose_name=_("Descrierea"))
    north = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_south", verbose_name=_("Nord"))
    northeast = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_southwest", verbose_name=_("Nord-est"))
    east = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_west", verbose_name=_("Est"))
    southeast = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_northwest", verbose_name=_("Sud-est"))
    south = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_north", verbose_name=_("Sud"))
    southwest = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_northeast", verbose_name=_("Sud-vest"))
    west = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_east", verbose_name=_("Vest"))
    northwest = models.ForeignKey("self", on_delete=models.SET_NULL, null=True, blank=True, related_name="to_southeast", verbose_name=_("Nord-vest"))

    class Meta:
        verbose_name = _("Județ")
        verbose_name_plural = _("Județe")

    def __str__(self):
        return self.name

    @property
    def neighbours(self):
        neighbour_counties = {}
        for field in self._meta.fields:
            if field.related_model == County:
                neighbour_counties[field.name] = field
        return neighbour_counties
    neighbours.fget.short_description = _("Vecinii")

在本例中，我想添加两个实例：Olt 和 Teleorman。如果我创建 Olt 并将 Teleorman 指定为其东部邻居，如何自动将 Teleorman 的西部邻居设置为 Olt？

我的想法是获取一个包含八个相邻字段作为属性的列表，它可以正常工作，直到我尝试在信号处理程序中获取字段实例。我需要反转邻居列表并获得最近更改的那个相反的值：如果我更改county1.east 的值，我需要一种方法如何将西作为应该在county2 中更新的字段。

pre_save 信号处理程序应该是什么样子？邻居@property 是否正确编写？

Answer 1

我看到解决此问题的一种方法是仅定义一半方向，并为另一半使用反向相关名称。但为了做到这一点，您需要将字段定义为OneToOneField：

class County(models.Model):
    north = models.OneToOneField(
        "self", on_delete=models.SET_NULL, 
        null=True, blank=True, 
        related_name="south"  # the other side
    )
    northeast = models.OneToOneField(
        "self", on_delete=models.SET_NULL, 
        null=True, blank=True, 
        related_name="southwest"  # the other side
    )
    east = models.OneToOneField(
        "self", on_delete=models.SET_NULL, 
        null=True, blank=True, 
        related_name="west"  # the other side
    )
    southeast = models.OneToOneField(
        "self", on_delete=models.SET_NULL, 
        null=True, blank=True, 
        related_name="northwest"  # the other side
    )

然后像这样使用它们：

c1 = County()
c2 = County()
c3 = County()

c1.north = c2  # this means c2.south == c1
c2.southeast = c3  # this means c3.northeast == c2

等等。这样您就不必担心设置方向的另一侧。此外，OneToOneField 还将确保一次只能有一个县位于另一个县的北部。