现有对象Django上的OneToMany

Question

我正在进行实验，以便更好地了解Django是如何工作的。下面的测试函数创建一个类别、标记和表示Macbook Pros的Item，然后将Macbook Pro添加到该类别、标记和项目(产品)中。

相关模型：(注意:我正在进行故障排除，模型可能不完全代表问题发生时的情况)

class Tag(models.Model):
    title = models.CharField(max_length=20)

    def __str__(self):
        return self.title

class Item(models.Model):
    name = models.CharField(max_length=100)
    description = models.CharField(max_length=100)
    manufacturer = models.ForeignKey('Manufacturer', blank=True, null=True, on_delete=models.SET_NULL)
    introduction = models.DateField(auto_now=True)
    quanity = models.IntegerField(default=0)
    is_retired = models.BooleanField(default=False)
    tags = models.ManyToManyField(Tag)

    def __str__(self):
        return self.name

    def add(self):
        pass

    def remove(self):
        pass

    def retire(self): # Rex came up with this, roll credits.
        pass 

    def count(self):
        pass

class OnHand(models.Model):
    name = models.CharField(max_length=100)
    serial = models.CharField(max_length=80)
    asset = models.CharField(max_length=20)
    product = models.ForeignKey(Item, blank=True, null=True, on_delete=models.CASCADE)


    def __str__(self):
        return self.serial

测试用例：

def make_macbook():
    """ Create a Manufacturer for Apple """
    apple = Manufacturer(name='Apple', rep_name='Jason Wilburn', support_number='1-800-Apple')
    apple.save()
    print('Manufacturer name: {}'.format(apple))

    """ Create a Category for Laptops """
    laptop = Category(name='Laptops')
    laptop.save()
    print('Category name: {}'.format(laptop))

    """ Create a Tag for Mac """
    mac = Tag(title='mac')
    mac.save()
    print('Tag name: {}'.format(mac))

    """ Create a MacBook Item """
    macbook = Item(name='Macbook Pro', description='15.5" Macbook Pro, 2018 Model', manufacturer=apple)
    macbook.save()
    print('Item name: {}'.format(macbook))

    """ Create a MacBook OnHand item """
    newMac = OnHand(name='MacBook Pro 15.5"', serial='SC02XP0NRJGH5', asset='DEPSC02XP0NRJGH5', product=macbook)
    newMac.save()


    return newMac.product

我现在测试的是将新的OnHand对象添加到类别、标记和项中。

以下是我所拥有的：

def add_macbook():
    """ Find MacBook Item """
    macbook = Item.objects.filter(name='Macbook Pro')

    """ Create a MacBook OnHand item """
    newMac = OnHand(name='MacBook Pro 15.5"', serial='000000000000000', asset='DEP0000000000000', product=macbook)
    newMac.save()

我面临两个问题：

1. 类“Item”没有“objects”成员
2. ValueError:无法分配"]>"："OnHand.product“必须是"Item”实例。

如何将现有的项实例关联起来？

我也尝试过：

def add_macbook():
    """ Find MacBook Item """
    macbook = Item.objects.get(name='Macbook Pro')

    """ Create a MacBook OnHand item """
    newMac = OnHand(name='MacBook Pro 15.5"', serial='000000000000000', asset='DEP0000000000000')
    newMac.save()
    macbook.product.add(newMac)

它向我展示了一个显而易见的'Item‘没有属性'product’，因为它显然没有。我尝试这是因为我的Tag模型是如何工作的。

Answer 1

.filter()函数返回一个查询集，而不是单个模型实例，您不能将整个查询集指定为外键。另一方面，.get()将返回一个模型实例。你几乎有了正确的组合。试试这个：

def add_macbook():
    """ Find MacBook Item """
    macbook = Item.objects.get(name='Macbook Pro')


    """ Create a MacBook OnHand item """
    newMac = OnHand(
        name='MacBook Pro 15.5"',
        serial='000000000000000',
        asset='DEP0000000000000',
        product=macbook
    )
    newMac.save()

Class 'Item' has no 'objects' member错误对我来说毫无意义。这可能只是IDE警告吗？

下面是过滤器函数的链接。