django.db.utils.IntegrityError: NOT NULL constraint failed: app.area_id

Question

When I run ./manage.py migrate,error happens django.db.utils.IntegrityError: NOT NULL constraint failed: app.area_id . models.py is

class Area(models.Model):
    name = models.CharField(max_length=20, verbose_name='area', null=True)

class User(models.Model):
    name = models.CharField(max_length=200,null=True)
    age = models.CharField(max_length=200,null=True)
    area = models.ForeignKey('Area', default="")

class Prefecture(models.Model):
    name = models.CharField(max_length=20, verbose_name='city')
    area = models.ForeignKey('Area')

class City(models.Model):
    name = models.CharField(max_length=20, verbose_name='region')
            prefecture = models.ForeignKey('Prefecture')

class Price(models.Model):
    name = models.CharField(max_length=20, verbose_name='price')
            PRICE_RANGE = (
                ('a', 'under500'),
                ('b', '500-1000'),
                ('c', 'upper1000'),
            )
    price_range = models.CharField(max_length=1, choices=PRICE_RANGE)
    city = models.ForeignKey('City')

When I wrote area = models.ForeignKey('Area'),I got an error You are trying to add a non-nullable field 'area' to transaction 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. What is wrong? How can I fix this?

Now models.py is

class Area(models.Model):
    name = models.CharField(max_length=20, verbose_name='area', null=True)

class User(models.Model):
    name = models.CharField(max_length=200,null=True)
    age = models.CharField(max_length=200,null=True)
    area = models.ForeignKey('Area', default="")

class Prefecture(models.Model):
    name = models.CharField(max_length=20, verbose_name='city')
    area = models.ForeignKey('Area', null=True, blank=True)

class City(models.Model):
    name = models.CharField(max_length=20, verbose_name='region')
            prefecture = models.ForeignKey('Prefecture', null=True, blank=True)

class Price(models.Model):
    name = models.CharField(max_length=20, verbose_name='price')
            PRICE_RANGE = (
                ('a', 'under500'),
                ('b', '500-1000'),
                ('c', 'upper1000'),
            )
    price_range = models.CharField(max_length=1, choices=PRICE_RANGE)
    city = models.ForeignKey('City', null=True, blank=True)

I also got same error.I already tried cache.clear() .

Answer 1

In the model below you are trying to add area as a ForeignKey. Since this Prefecture table has some data already Django does not know what to add in area field for existing rows.

class Prefecture(models.Model):
    name = models.CharField(max_length=20, verbose_name='city')
    area = models.ForeignKey('Area') # non null-able field Django does
                                     # not know what to add

Simple solution. Provide a default or add null

class Prefecture(models.Model):
        name = models.CharField(max_length=20, verbose_name='city')
        area = models.ForeignKey('Area', null=True)

Note: for all existing rows in Prefecture now area field will be null. You can add this field now for new rows.

Answer 2

You'll first need to delete the previously generated migration file.

Otherwise, even if you fix your models.py, you will keep getting the same error because ./manage.py migrate will keep trying to apply the previous migration.

After you delete the previously generated migration file, use the following:

area = models.ForeignKey('Area', blank=True, null=True)

This will make the area attribute optional, and you will be able to generate a new migration file and proceed with ./manage.py migrate.

After you've done that, you can optionally write a data migration script to fill in the missing area attributes. You can read more about in in the documentation.

You can also fill-in this attribute manually for your existing data, instead of writing a migration script (for instance, buy directly updating your database or by using django-admin).

Once you make sure that all your data has the area attribute filled-in, you can make your final change:

area = models.ForeignKey('Area')

When you apply this migration, you will simply add a NOT NULL constraint to this field in your database, and it should be OK if you filled-in the data in the previous step.