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 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

使用数据库工具检查数据库后，我意识到CategoryForeignKey 是NULL.&nbsp;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.