How to add a UniqueConstraint concurrently in Django

I spent some time searching for this answer before I realized I’d have to figure it out for myself. Now you don't have to. You’re welcome.

I came across a race condition that managed to sidestep my best efforts to keep a set of conditions and fields unique for one of my models. To solve this, decided to leverage Django’s UniqueConstraint:

class Baz(models.Model):
    foo = models.ForeignKey(Foo)
    bar = models.ForeignKey(Bar)
    is_active = models.Boolean()
    class Meta:
        constraints = [
            models.UniqueConstraint(
                conditions=Q(is_active=True)
                fields=["foo", "bar"],
                name="unique_active_foo_bar"
            )
        ]

After running ./manage.py makemigrations I got a migrations file that included this operation:

# migrations 0065
migrations.AddConstraint(
    model_name="bar",
    constraint=models.UniqueConstraint(
        condition=models.Q("is_active", True),
        fields=("foo", "bar"),
        name="unique_active_foo_bar",
    ),
),

This would be exactly what I wanted, except the table I’m migrating contains millions of rows. Under the hood, UniqueConstraint (for postgres) creates a UNIQUE INDEX meaning this would migration would lock and prevent any writes until the index was created. To avoid this, I wanted to apply the equivalent of migrations.addIndexConcurrently to my AddContraint. But after some searching around and reading the Django docs, I wasn’t able to find a prebuilt way to achieve this.

However, I did learn about ./manage.py sqlmigrate along the way. It would allow me to see the SQL my migration would generate. After running it on my migration with ./manage.py sqlmigrate app 0065 I saw the resulting SQL simply created a unique index:

CREATE UNIQUE INDEX "unique_active_foo_bar" ON "stock_job" ("foo", "bar") WHERE ("is_active" = true);

And after running ./manage.py sqlmigrate app 0065 --backwards I was able to get the SQL for the reserve migration:

DROP INDEX IF EXISTS "unique_active_foo_bar"

I then replaced the operation in my migration file with a SeparateDatabaseAndStatecommand so I could manage my state changes and database changes separately, and added myCONCURRENTLY requirement.

migrations.SeparateDatabaseAndState(
   database_operations = [
      migrations.RunSQL(
        sql="""
            CREATE UNIQUE INDEX CONCURRENTLY "unique_active_foo_bar"
            ON "app_baz" ("foo", "bar") WHERE ("is_active" = true);
        """,
        reverse_sql="""
            DROP INDEX CONCURRENTLY IF EXISTS
            "unique_active_foo_bar"
        """,
    ],    state_operations=[
       migrations.AddConstraint(
          model_name="bar",
          constraint=models.UniqueConstraint(
             condition=models.Q("is_active", True),
             fields=("foo", "bar"),
             name="unique_active_foo_bar",
          ),
       ),
    ],
)

I hope this saves someone just a little bit of time.