ForeignKey missing required positional argument on_delete when trying to create a Django migration

Question

I am trying to create tables using python/django and postgres. I create classes for each database table in models.py. When I try to create the migrations it gives me this errors:

File "..\database\models.py", line 40, in <module>
    class group(models.Model):
File "database\models.py", line 41, in group
    user_id = models.ForeignKey(user) TypeError: __init__() missing 1 required positional argument: 'on_delete'

This is my code

class user(models.Model):
    STATUS_CHOICES = (
        ('active','Active'),
        ('not active', 'Not active')
    )
    user_id= models.AutoField(primary_key=True)
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    email = models.EmailField(max_length=70, null=True, blank=True, unique=True)
    phone = models.CharField( max_length=17, blank=True)
    gender = models.CharField(max_length=50)
    region = models.CharField(max_length=50)
    password = models.CharField(widget=models.PasswordInput)
    active = models.CharField(max_length=50, choices=STATUS_CHOICES, default='not active')
    register_date = models.DateField(default=date.today)

    def _str_(self):
        return self.user_id

class train(models.Model):
    train_CHOICES = (
        ('intercity', 'Intercity'),
        ('sprinter', 'Sprinter')
    )
    train_id= models.AutoField(primary_key=True)
    train_type = models.CharField(max_length=50, choices=train_CHOICES, default='intercity')

    def _str_(self):
        return self.train_id

class group(models.Model):
    user_id = models.ForeignKey(user)
    group_id = models.AutoField(primary_key=True)
    trip_id = models.ForeignKey(trip)
    created_date = models.DateField(default=date.today)
    started_at = models.DateTimeField()

    def _str_(self):
        return self.group_id

I am really new to Python. I assume there are many mistakes. Could you please give a hint?

Answer 1

Since Django 2.0, on_delete is required argument for ForeignKey (in older versions it defaults to CASCADE), you should add it for all ForeignKey fields:

user_id = models.ForeignKey(user, on_delete=models.CASCADE)
trip_id = models.ForeignKey(trip, on_delete=models.CASCADE)

Note on_delete=models.CASCADE means that on deleting user or trip object Django deletes the objects containing the ForeignKey. You can find list of other options here.