Django implementation of default value in database

Question

I had a field on a model with was:

class SomeModel(models.Model):
    some_field = models.CharField(max_length=10, null=True, blank=True)

Then I changed my model to:

class SomeModel(models.Model):
    some_field = models.CharField(max_length=10, default='')

When I ran django-admin sqlmigrate somemodels somemigration to check my migration I found the following changes:

ALTER TABLE "somemodels" ALTER COLUMN "some_field" SET DEFAULT '';
UPDATE "somemodels" SET "some_field" = '' WHERE "some_field" IS NULL;
ALTER TABLE "somemodels" ALTER COLUMN "some_field" SET NOT NULL;
ALTER TABLE "somemodels" ALTER COLUMN "some_field" DROP DEFAULT;

I am not understanding why the Django apply a DROP DEFAULT in the table since I am creating a default value. If this is correct, how does Django implement the default values?

Information about my tools:

• Postgresql 9.5;
• Django 1.11b1;

Answer 1

The comments to django/db/backends/base/schema.py, starting ln. 571, detail the steps involved here:

When changing a column NULL constraint to NOT NULL with a given default value, we need to perform 4 steps:

1. Add a default for new incoming writes
2. Update existing NULL rows with new default
3. Replace NULL constraint with NOT NULL
4. Drop the default again.

Django does not usually use the built-in SQL default to set values (remember that Django can use callable values for defaults). You can find more information in this rejected bug report.