python - Django 的 on_delete 错误

Question

我目前正在做 Eric Matthes 所著的《Python 速成类(class)》一书中的一些项目。目前，我正在做实验 19.1，我必须使用以前的代码来制作 Django 项目，同时更改他自己的一些代码，但遇到了问题。每次我想运行这个命令

>>>python manage.py makemigration blogs

作为返回，我得到了这个代码

TypeError: init() missing 1 required positional argument: 'on_delete'

他原来的models.py代码:

from django.db import models

class Topic(models.Model):
    """A topic the user is learning about."""
    text = models.CharField(max_length=200)
    date_added = models.DateTimeField(auto_now_add=True) 

    def __str__(self):
        """Return a string representation of the model."""
        return self.text

class Entry(models.Model):
    """Something specific learned about a topic."""
    topic = models.ForeignKey(Topic)
    text = models.TextField()
    date_added = models.DateTimeField(auto_now_add=True)

    class Meta:
        verbose_name_plural = 'entries'

    def __str__(self):
        """Return a string representation of the model."""
        return self.text[:50] + "..."

和我当前的代码:

from django.db import models

# Create your models here.

class BlogPost(models.Model):
    title = models.CharField(max_length=200)
    text = models.CharField(max_length=200)
    date_added = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        """Return a string representation of the model."""
        return self.text

class Post(models.Model):
    """SThe post"""
    topic = models.ForeignKey(BlogPost)
    text = models.TextField()
    date_added = models.DateTimeField(auto_now_add=True)

    class Meta:
        verbose_name_plural = 'posts'

    def __str__(self):
        if len(self.text) >= 50:
            """Return a string represerntation of the model"""
            return self.text[:50] + "..."
        else:
            return self.text

老实说，我不知道为什么会收到此错误代码，我已经检查过是否弄乱了某些内容，但找不到任何内容。有谁知道吗？

Answer 1

自 Django 2.x 起，on_delete是必须的。这是来自 Django 2.0 发行说明:

The on_delete argument for ForeignKey and OneToOneField is now required in models and migrations. Consider squashing migrations so that you have fewer of them to update.

因此，错误来自这一行:

topic = models.ForeignKey(BlogPost)

您正在创建外键关系，但未提供 on_delete属性(property)。因此，请点击链接并选择适合您需求的一个。