Rails : migration for creating a fixed-length char(12) column

If Rails doesn’t understand the column type, it’ll pass it straight through to the database. So if you want a char instead of varchar, just replace:

t.column :token, :string

With:

t.column :token, "char(12)"

Of course, this may or may not make your migrations non-portable to another database.

(credit to http://laurelfan.com/2010/1/26/special-mysql-types-in-rails-migrations)