Django模型中的外键

Question

这里是我的情况： SubCategory有外键的Topic和Topic有外键SubCategory 。

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='+')

当我运行此代码时，它会给出以下错误：

NameError: name 'Topic' is not defined.

谁能告诉我如何解决它？

Answer 1

将Topic放在引号中： "Topic"

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

或者将Topic类放在SubCategory类之上

Answer 2

您的问题是在Subcategory之后声明Topic，因此在定义Subcategory时它不可用。

Python没有部分类声明或前向声明。 但是你可以使用init方法来实现类似的东西。 详情请见此处 。

Answer 3

尝试用引号括起模型：

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

您的问题有几种解决方案：您可以通过related_name ForeignKey属性定义后引用，从SubCategory到Topic ，或者只是将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='+')

    ...