Django模式中的外键

Question

这里是我的情况：SubCategory有Topic和Topic的外键有SubCategory的外键。

class SubCategory(models.Model): 
    name = models.CharField(max_length=100) 
    slug = models.SlugField(max_length=110) 
    description = models.TextField(default='') 
    ordering = models.PositiveIntegerField(default=1) 
    category = models.ForeignKey(Category) 
    created_on = models.DateTimeField(auto_now_add=True) 
    created_by = models.ForeignKey(User) 
    updated_on = models.DateTimeField(blank=True, null=True) 
    updated_by = models.ForeignKey(User, related_name='+') 
    num_topics = models.IntegerField(default=0) 
    num_posts = models.IntegerField(default=0) 
    last_topic = models.ForeignKey(Topic, related_name='+') 


class Topic(models.Model): 
    name = models.CharField(max_length=300) 
    slug = models.SlugField(max_length=300) 
    description = models.TextField(default='') 
    subcategory = models.ForeignKey(SubCategory) 
    created_on = models.DateTimeField(auto_now_add=True) 
    created_by = models.ForeignKey(User) 
    updated_on = models.DateTimeField(blank=True, null=True) 
    updated_by = models.ForeignKey(User, related_name='+') 

当我运行这段代码，它提供了以下错误：

NameError: name 'Topic' is not defined. 

任何人能告诉我如何解决它？

Answer 1

你的问题是Topic是在子类别之后声明的，因此它在定义子类别时不可用。

Python没有部分类声明或前向声明。但是您可以使用init方法来实现类似的功能。详情请参阅here。

Answer 2

要么把Topic引号："Topic"

last_topic = models.ForeignKey("Topic", related_name='+') 

或将Topic类SubCategory类

Answer 3

上面有几个解决你的问题：你既可以定义一个反向引用，从SubCategory到Topic通过related_nameForeignKey属性，或者只是将您的ForeignKey模型的名称作为字符串传递。

解决方案1：

class SubCategory(models.Model): 
    name = models.CharField(max_length=100) 
    slug = models.SlugField(max_length=110) 
    description = models.TextField(default='') 
    ordering = models.PositiveIntegerField(default=1) 
    ... 
    last_topic = models.ForeignKey(Topic, related_name='subcategory') 

解决方案2：

class SubCategory(models.Model): 
    name = models.CharField(max_length=100) 
    slug = models.SlugField(max_length=110) 
    description = models.TextField(default='') 
    ordering = models.PositiveIntegerField(default=1) 
    category = models.ForeignKey(Category) 
    created_on = models.DateTimeField(auto_now_add=True) 
    created_by = models.ForeignKey(User) 
    updated_on = models.DateTimeField(blank=True, null=True) 
    updated_by = models.ForeignKey(User, related_name='+') 
    num_topics = models.IntegerField(default=0) 
    num_posts = models.IntegerField(default=0) 
    last_topic = models.ForeignKey("Topic", related_name='+') 


class Topic(models.Model): 
    name = models.CharField(max_length=300) 
    slug = models.SlugField(max_length=300) 
    description = models.TextField(default='') 
    subcategory = models.ForeignKey(SubCategory) 
    created_on = models.DateTimeField(auto_now_add=True) 
    created_by = models.ForeignKey(User) 
    updated_on = models.DateTimeField(blank=True, null=True) 
    updated_by = models.ForeignKey(User, related_name='+') 

    ... 

Answer 4

尝试用引号引起型号：

class SubCategory(models.Model): 
    name = models.CharField(max_length=100) 
    slug = models.SlugField(max_length=110) 
    description = models.TextField(default='') 
    ordering = models.PositiveIntegerField(default=1) 
    category = models.ForeignKey('Category') 
    created_on = models.DateTimeField(auto_now_add=True) 
    created_by = models.ForeignKey('User') 
    updated_on = models.DateTimeField(blank=True, null=True) 
    updated_by = models.ForeignKey('User', related_name='+') 
    num_topics = models.IntegerField(default=0) 
    num_posts = models.IntegerField(default=0) 
    last_topic = models.ForeignKey('Topic', related_name='+') 


class Topic(models.Model): 
    name = models.CharField(max_length=300) 
    slug = models.SlugField(max_length=300) 
    description = models.TextField(default='') 
    subcategory = models.ForeignKey("SubCategory") 
    created_on = models.DateTimeField(auto_now_add=True) 
    created_by = models.ForeignKey('User') 
    updated_on = models.DateTimeField(blank=True, null=True) 
    updated_by = models.ForeignKey('User', related_name='+')