根据特定类别名称过滤对象

Question

我正在尝试过滤类别名称为“书籍”的所有产品。请参阅下面我尝试过的内容，但我得到一个空的查询集。这是我models.py的以防万一：from django.db import modelsfrom django.urls import reverse[...]      class Product(models.Model):    title = models.CharField(max_length=200)    description = models.TextField()    price = models.DecimalField(decimal_places=2, max_digits=10)    year = models.PositiveSmallIntegerField()    image = models.ImageField(upload_to=get_image_path, blank=True, null=True)    date_added = models.DateTimeField(auto_now=True, null=True)    category = models.ForeignKey('Category', on_delete=models.CASCADE)    [...]class Category(models.Model):    parent = models.ForeignKey('self', null=True, blank=True, on_delete=models.CASCADE)    name = models.CharField(max_length=255)    def __str__(self):        return self.name我试过这个：>>> from products.models import Product, Category>>> c = Category(name='Books')>>> c1 = Category(name='Textbooks', parent=c)>>> c2 = Category(name='Primary school', parent=c1)>>> p = Product(title='spam', description='spameggs', price=200, year=2018, category=c2)>>> for x in (c, c1, c2, p):...     x.save()... >>> Product.objects.get(category=c2)<Product: Product object (1)>>>> Product.objects.filter(category__name='Books')<QuerySet []>>>> Product.objects.filter(Q(category__name='Books') | Q(category__parent__name='Books') | Q(category__parent__parent__name="Books"))<QuerySet []>

Answer 1

使用数据库工具检查数据库后，我意识到CategoryForeignKey 是NULL. 问题是我在保存父母之前分配了父母，这意味着他们没有 PrimaryKey，所以 ForeignKey 设置为NULL.