Web Frameworks

This page serves a reference guide for common tasks within web frameworks. Squawk doesn't have special support for Django or any other web ORM. This page is merely a guide.

Django ORM#

Django can auto generate schema changes, but many times the generated SQL won't pass Squawk's rules. These custom database backends for zero downtime migrations might work for your use case:

making a field non-nullable#

In the Django ORM, making a nullable field non-nullable requires care to make the change safe.

This example shows how to make a nullable boolean field a non-nullable field.

# before
class Post(Model):
published = models.BooleanField(null=True)
# after
class Post(Model):
published = models.BooleanField(null=False, default=False)

migration steps#

We cannot use the auto generated SQL from Django's migration system. We must use django.db.migrations.operations.SeparateDatabaseAndState to make changes to our Django models while using custom SQL.

  1. Update the model with a default so newly created objects in Django get assigned a default value. This change must be deployed before making the SQL migrations.

    class Post(Model):
    published = models.BooleanField(default=False)
  2. Create trigger to set default value false for inserts published is null.

    CREATE FUNCTION null_published_trigger()
    RETURNS trigger AS '
    IF NEW.published IS NULL THEN
    NEW.published := false;
    END IF;
    END' LANGUAGE 'plpgsql';
    CREATE TRIGGER published_default_trigger
    EXECUTE PROCEDURE null_published_trigger();
  3. Set a default value for existing objects.

    UPDATE "post" SET "published" = false WHERE "published" IS NULL;
  1. Create a not null constraint.

    ALTER TABLE "post" ADD CONSTRAINT published_not_null
    CHECK ("published" IS NOT NULL) NOT VALID;
  2. Validate constraint.

    ALTER TABLE "post" VALIDATE CONSTRAINT published_not_null;
  3. Remove trigger.

    DROP TRIGGER IF EXISTS published_default_trigger on post;
    DROP FUNCTION IF EXISTS null_published_trigger;