db: person_id can only be updated if index is updated too.

This fixes the following error:

Mysql2::Error: Duplicate entry '0' for key
'index_profiles_on_person_id': ALTER TABLE `profiles` CHANGE `person_id`
`person_id` int(11) NOT NULL

db/migrate/20110127000953_make_fields_not_null.rb

@@ -20,11 +20,13 @@ class MakeFieldsNotNull < ActiveRecord::Migration
   end
 
   def self.up
+    remove_index(:profiles, :person_id)
     non_nullable_fields.each_pair do |table, columns|
       columns.each do |column|
         change_column_null(table, column, false)
       end
     end
+    add_index :profiles, :person_id
   end
 
   def self.down