Migration fails when removing a NULL constraint

[Markus Holtermann]

If a model has e.g IntegerField(null=True) and a migration changes it to IntegerField(default=42) the migration fails on SQLite due to the way the new table is created (create new table and copy old data into it) and PostgreSQL due to no default being used for existing rows. The respective SQL code Django produces is:

-- SQLite
CREATE TABLE "testapp_foo__new" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "d" integer NOT NULL);
INSERT INTO "testapp_foo__new" ("id", "d") SELECT "id", "d" FROM "testapp_foo";
DROP TABLE "testapp_foo";
ALTER TABLE "testapp_foo__new" RENAME TO "testapp_foo";

-- PostgreSQL
ALTER TABLE "testapp_foo" ALTER COLUMN "d" SET DEFAULT 42, ALTER COLUMN "d" SET NOT NULL;
ALTER TABLE "testapp_foo" ALTER COLUMN "d" DROP DEFAULT;

The first idea for a patch was to do a two-step insert, first all rows w/o NULL values and second those w/ NULL and adding a the default explicitly. This would involve some more code changes. apollo13 came up with the idea to use a CASE WHEN SQL syntax which, as it turns out, seems quite nice. jarshwah came up with coalesce("d", 42).

-- SQLite
CREATE TABLE "testapp_foo__new" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "d" integer NOT NULL);
INSERT INTO "testapp_foo__new" ("id", "d") SELECT "id", coalesc("d", 42) FROM "testapp_foo";
DROP TABLE "testapp_foo";
ALTER TABLE "testapp_foo__new" RENAME TO "testapp_foo";

-- PostgreSQL
ALTER TABLE "testapp_foo" ALTER COLUMN "d" SET DEFAULT 42;
UPDATE "testapp_foo" SET "d" = 42 WHERE "d" IS NULL;
ALTER TABLE "testapp_foo" ALTER COLUMN "d" SET NOT NULL;
ALTER TABLE "testapp_foo" ALTER COLUMN "d" DROP DEFAULT;

[Markus Holtermann]

I added a pull-request: ​https://github.com/django/django/pull/3320

[Simon Charette]

Great patch. Left some comments on the PR.

[Markus Holtermann]

Updated description after discussion with jarshwah and charettes