http://blog.carbonfive.com/2011/01/27/start-testing-your-migrations-right-now/

alert.rb

class Alert < ActiveRecord::Base
  def self.delete_stale
    execute <<-EOS
    DELETE FROM alerts
     WHERE alerts.alertable_type = 'Post'
     AND alerts.alertable_id IN (
      SELECT id FROM posts WHERE deleted = 1
     )
    EOS
  end
end

alert_test.rb

# Use normal testing code since this is just a unit test now...

migration.rb

 class DeleteAlertsForDeletedPosts < ActiveRecord::Migration
  def self.up
    Alert.delete_stale
  end

  def self.down
    say "Previous migration removed stale alerts, there is nothing to do in this .down"
  end
end

My issue with this is pointed out below:
http://gem-session.com/2010/03/how-to-use-models-in-your-migrations-without-killing-kittens

Since the resolution would be to move the code to the migration itself (ie: by reopening the Alert model and adding the delete_stale method within your migration .rb) it seems like the test focus moves back from the model and into the migration.

I follow where you're going with this though, and still find that this gray area of data migration (and not schema change) is not a well defined best practice. What do you think?