# Reading the SQL Behind a Migration — Django / FastAPI

Source: https://www.skillbyai.com/en/django-fastapi/m-sql

> Know what will run.

## sqlmigrate shows the statements

`python manage.py sqlmigrate app 0001` prints the SQL a migration will execute for your database backend, without running it. Check it for large tables: adding indexes or NOT NULL columns can lock tables for a long time. On PostgreSQL, Django offers operations such as AddIndexConcurrently for zero-downtime index creation.

## The SQL for the initial migration, run

I ran this with Django 6.1.1 and Python 3.12 in a demo project (shopsite with a catalog app) using SQLite. SQLite statements create both tables with a unique category name, a foreign key, the index Django adds for the foreign key, and the composite index from Meta.

```bash
python manage.py sqlmigrate catalog 0001
```

Output:

```
BEGIN;
--
-- Create model Category
--
CREATE TABLE "catalog_category" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "name" varchar(50) NOT NULL UNIQUE);
--
-- Create model Product
--
CREATE TABLE "catalog_product" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "name" varchar(100) NOT NULL, "price" decimal NOT NULL, "in_stock" bool NOT NULL, "created" datetime NOT NULL, "category_id" bigint NOT NULL REFERENCES "catalog_category" ("id") DEFERRABLE INITIALLY DEFERRED);
CREATE INDEX "catalog_product_category_id_35bf920b" ON "catalog_product" ("category_id");
CREATE INDEX "catalog_pro_categor_5a4a66_idx" ON "catalog_product" ("category_id", "price");
COMMIT;
```

## Run sqlmigrate on large-table changes

Reviewing the generated SQL is the cheapest way to spot a migration that would lock production.

**Quiz:** What does sqlmigrate do?

- [ ] Applies all migrations
- [x] Prints the SQL for a migration without running it
- [ ] Deletes migrations
- [ ] Creates a superuser

*Answer:* Prints the SQL for a migration without running it. Inspect before applying.