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;

brianpck · Accepted Answer · 2017-04-17 18:43:28Z

7

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:

Add a default for new incoming writes

Update existing NULL rows with new default

Replace NULL constraint with NOT NULL

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.

answered Apr 17, 2017 at 18:43

brianpck

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2 Comments

lucastamoios Over a year ago

Thank you a lot! In the bug report you posted, someone still says "Django uses database defaults to set values on existing rows in a table. It doesn't leave the default values in the database so dropping the default is correct behavior.". Seems some sort of Django's optimization.

brianpck Over a year ago

@lucastamoios I'm not positive...that seems to be different from the first bullet point in the comment, which seems to be just a safeguard for incoming values during the migration...

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