基于特定类别过滤对象

Question

我正在尝试筛选所有类别名称为“Books”的产品。看看下面我尝试了什么，但我得到了一个空的查询集

这是我的models.py，以防万一：

from django.db import models
from 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

在使用数据库工具检查数据库之后，我意识到CategoryForeignKey是NULL。问题是我在保存父级之前分配了父级，这意味着它们没有PrimaryKey，所以ForeignKey被设置为NULL