Auto increment a non-primary key field in Ruby on Rails

In a RoR migration, how do I auto increment a开发者_如何学运维 non-primary-key field? I'd like to do this in the db definition, and not in the model.

You need to execute an SQL statement.

statement = "ALTER TABLE `users` CHANGE `id` `id` SMALLINT( 5 ) UNSIGNED NOT NULL AUTO_INCREMENT" 
ActiveRecord::Base.connection.execute(statement)

you can entry manually in your migration

Note this is just an example. The final SQL statement syntax depends on the database.