PJ Hyett has created an impressive plugin for Ruby on Rails, called auto_migrations.
It deduces changes between the db/schema.rb and the current database schema, and executes the appropriate commands to bring the latter in sync with the former. It supports creating tables, adding, changing, and removing fields, adding and removing indexes. It also allows you to create the real migrations based on schema changes.
Read more at author’s blog: Err the Blog: “Automatically”.