CODEWITHSUNDEEP
djangoforeign-keyscs50
Walsh259
Walsh259

Reputation: 39

Django ForeignKey value does not have corresponding value

I'm working on the Django section of CS50 around the 30min mark on the video with the modes 'ForeignKey' section.

When i run the make migration i get the error.

You are trying to add a non-nullable field 'agent' to product without a default; we can't do that (the database needs something to populate existing rows).

Please select a fix:

1) Provide a one-off default now (will be set on all existing rows with a null value for this column)

2) Quit, and let me add a default in models.py

Select an option: 2

if I set the default as a string value i get the error saying it was expecting an id.

If i set it as 1 then i get the following.

The row in table 'clutter_product' with primary key '1' has an invalid foreign key: clutter_product.agent_id contains a value '1' that does not have a corresponding value in clutter_supplier.id.

class Supplier(models.Model):
company = models.CharField(max_length=64)
contact = models.CharField(max_length=64)
email = models.CharField(max_length=128, blank = True)

def __str__(self):
    return f"{self.id} {self.company} {self.contact} {self.email}"

class Product(models.Model):
    name = models.CharField(max_length=64)
    sku = models.CharField(max_length=64, blank = True)
    unit_cost = models.IntegerField()
    rrp = models.IntegerField()
    average_fee = models.IntegerField()
    shipping_fee = models.IntegerField()
    agent = models.ForeignKey(Supplier, default=1, on_delete=models.CASCADE, related_name="suppliers")

Upvotes: 0

Views: 3232

Answers (1)

ruddra
ruddra

Reputation: 51988

I would suggest to try these steps:

First, delete the migration file which is causing this problem.

Second change the model like this:

class Product(models.Model):
    # rest of the fields
    agent = models.ForeignKey(Supplier, null=True, default=None, on_delete=models.CASCADE, related_name="suppliers")

Third run makemigrations and migrate.

Fourth create a instance of supplier using supplier = Supplier.objects.create(company="ABC", contact="contact", email="[email protected]", pk=1)

Fifth(optional) Update the existing Product to haven a supplier-Product.objects.update(agent=supplier).

Sixth, if you want to constrain Products to be created with an agent, then remove null=True, default=None from agent field in Product model. Then run makemigrations and migrate.

Upvotes: 4

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