CODEWITHSUNDEEP
pythondjangodjango-models
tices
tices

Reputation: 96

What do I set the default value as when adding a field to a model?

I added the field user which is a foreign key to another model called User. This field was added to the model called Bid. However, when I tried to migrate the changes, I got the message:

It is impossible to add a non-nullable field 'user' to bid without specifying a default. This is because 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 manually define a default value in models.py.

Last time, I set it to 'user' and got an error that stated: ValueError: invalid literal for int() with base 10: 'user'.

What should I set the default value as?

models.py:

class Bid(models.Model):
    item = models.ForeignKey(Listing, on_delete=models.CASCADE)
    price = models.FloatField()
    user = models.ForeignKey(User, on_delete=models.CASCADE)

Upvotes: 1

Views: 787

Answers (2)

Richard Neumann
Richard Neumann

Reputation: 3381

If you add a relationship via a new foreign key, you have two options

  1. You make the new FK nullable and let it default to NULL (i.e. None in python). This way, legacy entries with unknown relations will have NULL as the FK, i.e. do not know their users.
  2. You manually populate the legacy fields with the appropriate foreign keys to the Bid records. This requires that you have kept that information beforehand.

Upvotes: 1

Sorin Burghiu
Sorin Burghiu

Reputation: 779

As the error says, you are creating a new field in your table. When creating a new field, existing rows need to be taken into consideration. Safest approach is to set it as null=True and handle the field later.

user = models.ForeignKey(User, on_delete=models.SET_NULL, null=True)

However you may not want a Bid to have a null user. In which case I recommend looking into how to write a custom migration to populate existing rows.

Another note: if the data you have in your table is not needed, you could consider dropping the table and rebuilding it or deleting your migrations and creating them again with manage.py makemigrations - again only if the data you have in your db is not needed.

Upvotes: 1

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