Migration fails on SQLite 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). The respective SQL code Django produces is:

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";

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:

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", CASE WHEN "d" IS NULL THEN 42 ELSE "d" END FROM "testapp_foo";