django-models - 如何修复 '' Follow.s_id' 不是 'Users' 的外键。”在此模型中

Question

我是 Django 框架的新手，我正在尝试构建以下系统，其中包含一个关注多个股票的用户，并且这些股票被多个人关注，我正在尝试使用中间类创建一个复合键，但是我遇到了这个错误。

class Stock(models.Model):
    symbol = models.CharField(max_length=12, primary_key=True,default="")
    name = models.CharField(max_length=64)
    top_rank = models.IntegerField(null=True)
    price = models.FloatField()
    change = models.FloatField(null=True)
    change_percent = models.FloatField()
    market_cap = models.FloatField(null=True)
    primary_exchange = models.CharField(null=True, max_length=32) # NASDAQ
    followers = models.ManyToManyField('myapp.Users',through='Follow',through_fields=('u_id','s_id'))


class Users(models.Model):
    user_id = models.IntegerField(primary_key=True,default="")
    name = models.CharField(max_length=12)

class Follow(models.Model):
    u_id=models.ForeignKey('Users',on_delete=models.CASCADE)
    s_id=models.ForeignKey('Stock',on_delete=models.CASCADE)

myapp.Stock.followers: (fields.E339) 'Follow.s_id' is not a foreign key to 'Users'.
        HINT: Did you mean one of the following foreign keys to 'Users': u_id?
myapp.Stock.followers: (fields.E339) 'Follow.u_id' is not a foreign key to 'Stock'.
        HINT: Did you mean one of the following foreign keys to 'Stock': s_id?

Answer 1

through_fields [Django-doc] 的顺序是不正确的。正如文档中所指定的:

through_fields accepts a 2-tuple ('field1', 'field2'), where field1 is the name of the foreign key to the model the ManyToManyField is defined on (...), and field2 the name of the foreign key to the target model (...).

因此，这意味着 through_fields 的第一项应该是 's_id'，因为它引用的是 Stock 模型，即模型您在其中定义了 ManyToManyField，第二项应为 'u_id':

class Stock(models.Model):
    # …
    followers = models.ManyToManyField(
        'myapp.Users',
        through='Follow',
        through_fields=(<b>'s_id','u_id'</b>)
    )

话虽这么说，您不需要在这里定义 through_fields，因为您的 Follow 模型恰好包含两个 ForeignKey 指向不同的模型。事实上，您也不需要定义 through=... 模型，因为它不包含任何额外的字段。

请注意，通常 ForeignKey 不包含 _id 后缀，因为 Django 会自动添加一个带有 _id 的额外字段 后缀，包含引用值的主键。

因此，将模型定义为:

class Stock(models.Model):
    symbol = models.CharField(max_length=12, primary_key=True)
    name = models.CharField(max_length=64)
    top_rank = models.IntegerField(null=True)
    price = models.FloatField()
    change = models.FloatField(null=True)
    change_percent = models.FloatField()
    market_cap = models.FloatField(null=True)
    primary_exchange = models.CharField(null=True, max_length=32) # NASDAQ
    followers = <b>models.ManyToManyField('myapp.User')</b>


class <b>User</b>(models.Model):
    user_id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=12)

请注意，模型通常具有单数名称，即User，而不是Users。