ManyToMany字段尽管有值但仍返回None

Question

我有2个班级：类别和比萨饼。 其中一个是对象的类别列表，另一个是对象。 问题是，当我调用一个对象时，我可以看到所有字段，除了ManyToMany字段返回catalog.Category.None而不是分配的值。

这是我的models.py：

class Category(models.Model):
    CLASSIC = 'Classic'
    VEGETARIAN = 'Vegetarian'
    SPICY = 'Spicy'
    TYPE = [
        (CLASSIC, 'Classic'),
        (VEGETARIAN, 'Vegetarian'),
        (SPICY, 'Spicy')
    ]

    type = models.CharField(
        max_length=100,
        choices=TYPE
    )

    def __str__(self):
        return self.type

    class Meta:
        verbose_name_plural = 'Categories'


class Pizza(models.Model):
    name = models.CharField(max_length=100)
    price = models.IntegerField()
    size = models.OneToOneField(
        Size,
        on_delete=models.CASCADE,
        primary_key=True
    )
    category = models.ManyToManyField(Category)

    def __str__(self):
        return f"name = {self.name}, size = {self.size}, category = {self.category}"

这是我的shell输出：

>>> from catalog.models import Pizza
>>> from catalog.models import Category
>>> pizza = Pizza.objects.all()
>>> pizza
<QuerySet [<Pizza: name = Chicken, prices = 8, 10, category = catalog.Category.None>, <Pizza: name = Pepperoni, prices = 8, 12, category = catalog.Category.None>, <Pizza: name = Mushroom, prices = 7, 9, category = catalog.Category.None>]>
>>> cat = Category.objects.filter(pizza=1)
>>> cat
<QuerySet [<Category: Classic>]>
>>> cat = Category.objects.filter(pizza=2)
>>> cat
<QuerySet [<Category: Classic>, <Category: Spicy>]>

Answer 1

some_pizza.category实际上将返回catalog.Category.None ，因为这是ManyRelatedManager ，并且是在ManyRelatedManager上str(..)的预期输出。 您应该添加.all()以将其呈现为Category的列表：

class Pizza(models.Model):

    # ...

    def __str__(self):
        return f"name = {self.name}, size = {self.size}, category = {self.category.all()}"

注意 ：由于Pizza可以具有零个，一个或多个类别，因此最好将ManyToManyField categories命名为而不是category 。