autogenerated table for ManyToMany relation does not include UNIQUE constraint if any other field is changed

[Yevhen Kozlov]

0. create a models.py, makemigrations for them

from django.db import models

class Test(models.Model):
	pass

class Test2(models.Model):
	title = models.CharField(max_length=50)

1. add manyToMany field only, makemigrations:

from django.db import models

class Test(models.Model):
	pass

class Test2(models.Model):
	title = models.CharField(max_length=50)
	tests = models.ManyToManyField(Test)

Check with sqlmigrate there is table ..._test2_test with UNIQUE INDEX:

CREATE TABLE "m2m_test2_tests" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "test2_id" integer NOT NULL REFERENCES "m2m_test2" ("id"), "test_id" integer NOT NULL REFERENCES "m2m_test" ("id"));
CREATE UNIQUE INDEX "m2m_test2_tests_test2_id_test_id_c4708311_uniq" ON "m2m_test2_tests" ("test2_id", "test_id");
CREATE INDEX "m2m_test2_tests_test2_id_de2c1678" ON "m2m_test2_tests" ("test2_id");
CREATE INDEX "m2m_test2_tests_test_id_9723a1f8" ON "m2m_test2_tests" ("test_id");

2. drop migration created at step 1 and change title field additionally(say, set max_length=100)

from django.db import models

class Test(models.Model):
	pass

class Test2(models.Model):
	title = models.CharField(max_length=50)
	tests = models.ManyToManyField(Test)

Then run makemigrations and check its output:

BEGIN;
--
-- Add field tests to test2
--
CREATE TABLE "m2m_test2_tests" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "test2_id" integer NOT NULL REFERENCES "m2m_test2" ("id"), "test_id" integer NOT NULL REFERENCES "m2m_test" ("id"));
--
-- Alter field title on test2
--
ALTER TABLE "m2m_test2" RENAME TO "m2m_test2__old";
CREATE TABLE "m2m_test2" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "title" varchar(50) NOT NULL);
INSERT INTO "m2m_test2" ("id", "title") SELECT "id", "title" FROM "m2m_test2__old";
DROP TABLE "m2m_test2__old";
COMMIT;

So no UNIQUE constraint is generated for automatically created table if there is any other field changed in the same migration.