Migrations are awesome magic, until the magic fails. Then it's an awesomely frightening engine of change that threatens to wipe out your entire database.
So, before doing what I say, backup your database.
I had to enforce a "unique=True" constraint on a model. Migrations are supposed to automatically create a constraint in SQL, as well as its implied index. A quick look at the indexes (SHOW INDEXES FROM Model) didn't show the index.