如何限制 Django ORM 中外键关系的数量？

Question

我有一个与自身递归相关的模型。其内容如下。

class Category(models.Model):
    name = models.CharField(max_length=200)
    parent = models.ForeignKey(
        'self', blank=True, null=True,
        related_name='children', on_delete=models.CASCADE)

    class Meta:
        verbose_name_plural = "Categories"

    def __str__(self):
        path = [self.name, ]
        node = self.parent
        while node is not None:
            path.append(node.name)
            node = node.parent
        return '->'.join(path[::-1])

我想要实现的是一个小验证，防止超过 3 个孩子进入父类别。 例如，如果我按照上面的状态创建，它将保存如下：

Programming->Back-end->Python->ifelse.

我的问题：如何防止父类别有超过 3 个孩子？

Answer 1

你可以试试model validation

from django.core.exceptions import ValidationError


class Category(models.Model):
    name = models.CharField(max_length=200)
    parent = models.ForeignKey(
        'self', blank=True, null=True,
        related_name='children', on_delete=models.CASCADE)

    def clean(self):
        if self.parent:
            if self.get_parents_length(self) > 3:
                raise ValidationError("Parent has already got 3 children!")

    class Meta:
        verbose_name_plural = "Categories"

    def __str__(self):
        path = [self.name, ]
        node = self.parent
        while node is not None:
            path.append(node.name)
            node = node.parent
        return '->'.join(path[::-1])

    @staticmethod
    def get_parents_length(cat):
        parents = [cat.name]
        node = cat.parent
        while node is not None:
            parents.append(node.name)
            node = node.parent
        return len(parents)