Django: IntegrityError when migrating after adding foreign key field - django

Question

I have three models as follows in a Django app named markets:
class Market(models.Model):
title = models.CharField(max_length=50, default="")
current_price = models.DecimalField(max_digits=5, decimal_places=2, default=0.50)
description = models.TextField(default="")
shares_yes = models.IntegerField(default=0)
shares_no = models.IntegerField(default=0)
b = models.IntegerField(default=100)
cost_function = models.IntegerField(default=0)
open = models.BooleanField(default=True)
def __str__(self):
return self.title[:50]
def get_absolute_url(self):
return reverse('market_detail', args=[str(self.id)])
class Price(models.Model):
market = models.ForeignKey(
Market,
on_delete=models.CASCADE,
related_name='prices',
default=None)
price = models.DecimalField(
max_digits=5,
decimal_places=2,
default=0.50)
price_date = models.DateTimeField(
default=now,
blank=True)
def __str__(self):
return str(self.price)
def get_absolute_url(self):
return reverse('market_list')
class Share(models.Model):
user = models.ForeignKey('users.CustomUser',
on_delete=models.CASCADE,
related_name='user_shares',
default=None)
market = models.ForeignKey(
Market,
on_delete=models.CASCADE,
related_name='market_shares',
default=None)
share = models.IntegerField(default=0)
transaction_date = models.DateTimeField(
default=now,
blank=True)
def __str__(self):
return str(self.share)
def get_absolute_url(self):
return reverse('market_list')
I would like to add the following foreign key field to the Price model:
user = models.ForeignKey('users.CustomUser',
on_delete=models.CASCADE,
related_name='user_prices',
default=None)
When I run makemigrations on markets, there's no issue. But when I try to actually migrate the database, I get the following error:
django.db.utils.IntegrityError: column "user_id" contains null values
Why is that? I had no issues adding a user field to the Share model, so am not clear on why I run into problems when also looking to add it to Price.

Answer 1

When I run makemigrations on markets, there's no issue. But when I try
to actually migrate the database, I get the following error:
django.db.utils.IntegrityError: column "user_id" contains null values
Why is that?
A ForeignKey is by default non-NULLable. But you specify a default=None. The migraiton thus aims to insert NULL for the existing records, and that will fail.
You can make your ForeignKey nullable with:
user = models.ForeignKey('users.CustomUser',
on_delete=models.CASCADE,
related_name='user_prices',
null=True,
default=None
)
You will need to remove (or alter) the migration file, and recreate a migration file.
Another way to resolving this is providing a CustomUser object to which you link the existing records, with that user.
Note: usually it is better to use get_user_model() [Django-doc] instead of providing the user model as a reference or string. If you later change your mind, you can alter the setting, and then all ForeignKeys will be remapped on the new user model.