多对多关系上的Django makemigrations错误

Question

我想要什么：

存储有关人群运行的信息。

我做了什么：

from django.db import models
from django.contrib.auth.models import User
from datetime import timedelta


class Route(models.Model):
    name = models.CharField(max_length=50)


class Run(models.Model):
    date = models.DateField()
    type = models.ForeignKey(Route, on_delete=models.PROTECT)
    runners = models.ManyToManyField(User, through='RunnerResult', through_fields=["user", "run"])


class RunnerResult(models.Model):
    user = models.ForeignKey(User, on_delete=models.PROTECT)
    run = models.ForeignKey('Run', on_delete=models.PROTECT)
    result = models.DurationField(default=timedelta())

问题：

当我执行makemigrations 时，出现以下错误：

SystemCheckError: System check identified some issues:

ERRORS:
run.Run.runners: (fields.E339) 'RunnerResult.run' is not a foreign key to 'User'.
        HINT: Did you mean one of the following foreign keys to 'User': user?
run.Run.runners: (fields.E339) 'RunnerResult.user' is not a foreign key to 'Run'.
        HINT: Did you mean one of the following foreign keys to 'Run': run?

试图通过_fields 和模型在彼此之间以及其他一些操作之间交换。我开始思考我对 M2M 关系的误解。

Answer 1

您在错误的顺序中指定了through_fields。您首先应该指定引用 source 的关系，然后再指定引用 target 的关系，所以：

class Run(models.Model):
    date = models.DateField()
    type = models.ForeignKey(Route, on_delete=models.PROTECT)
    runners = models.ManyToManyField(
        User,
        through='RunnerResult',
        through_fields=('run', 'user')
    )

由于ForeignKey 到Run 和User 只有一个，因此您不需要指定through_fields=… parameter [Django-doc]。因此，您可以将其实现为：

class Run(models.Model):
    date = models.DateField()
    type = models.ForeignKey(Route, on_delete=models.PROTECT)
    runners = models.ManyToManyField(
        User,
        through='RunnerResult'
        # no through_fields
    )

---

注意：通常使用settings.AUTH_USER_MODEL [Django-doc] 引用用户模型比直接使用User model [Django-doc] 更好。更多信息可以查看referencing the User model section of the documentation。