I have a Django model that used to look like this:class Car(models.Model): manufacturer_id = models.IntegerField()There is another model called Manufacturer that the id field refers to. However, I realized that it would be useful to use Django's built-in foreign key functionality, so I changed the model to this:class Car(models.Model): manufacturer = models.ForeignKey(Manufacturer)This change appears to work fine immediately, queries work without errors, but when I try to run migrations, Django outputs the following:- Remove field manufacturer_id from car - Add field manufacturer to carDoing this migration would clear all the existing relationships in the database, so I don't want to do that. I don't really want any migrations at all, since queries like Car.objects.get(manufacturer__name="Toyota") work fine. I would like a proper database foreign key constraint, but it's not a high priority.So my question is this: Is there a way to make a migration or something else that allows me to convert an existing field to a foreign key? I cannot use --fake since I need to reliably work across dev, prod, and my coworkers' computers.内容来源于 Stack Overflow, 遵循 CCBY-SA 4.0 许可协议进行翻译与使用。原文链接：Django change an existing field to foreign key

Python的__str__()方法说明：本文章的Python代码测试基于Python3.x__str__()方法的作用：不定义__str__方法，print输出对象实例时，默认打印对象实例的内存地址定义__str__方法，print输出对象实例时，自动调用此方法返回值必须是字符串在Python中，方法名形如__xxx__()的方法表示此方法具有特殊的功能，也称为魔法方法验证1. 不定义__str__方法，print输出对象实例时，默认打印的是对象的内存地址# 创建学生类 class Stu(): def __init__(self, name, age): self.name = name self.age = age s = Stu("张三", 18) print(s)输出：<__main__.Stu object at 0x000002526AF05A60>2. 定义__str__方法，print输出对象实例时，自动调用此方法class Stu(): def __init__(self, name, age): self.name = name self.age = age def __str__(self): return "学生" + self.name + "的年龄是" + str(self.age) s = Stu("张三", 18) # 打印对象实例 print(s) 输出：学生张三的年龄是183. 返回值必须是字符串class Stu(): def __init__(self, name, age): self.name = name self.age = age def __str__(self): return self.age s = Stu("张三", 18) print(s)输出：--------------------------------------------------------------------------- TypeError Traceback (most recent call last) 7 8 s = Stu("张三", 18) ----> 9 print(s) TypeError: __str__ returned non-string (type int)因为__str__()返回值非字符串，所以报错TypeError: __str__ returned non-string (type int)原文链接：https://blog.csdn.net/what_how_why2020/article/details/114748495