Use the execute command (outside the create table loop) to inject SQL statements for whatever isn't covered by the migration syntax (which is limited). For example:
execute "ALTER TABLE items ADD COLUMN cost_unit DECIMAL(5,2) NOT NULL DEFAULT 0"
Also check out the foreign_key_migrations plugin. (http://www.redhillconsulting.com.au/rails_plugins.html). I use it to handle relationships automatically.
