Migration

class CreateMedia < ActiveRecord::Migration
  def self.up
    create_table :media do |t|
      t.string :name, :null => false
      t.text :description
      t.references :asset, :null => false

      t.timestamps
    end
    
    execute <<-EOS
      ALTER TABLE media
        ADD FOREIGN KEY (asset_id)
            REFERENCES assets (id)
              ON DELETE CASCADE
                ON UPDATE CASCADE;
    EOS
    
  end

  def self.down
    drop_table :medias
  end
end

Spec

# Delete ignores AR callbacks
it "should cascade delete the asset if deleted" do
  asset = @media.asset   

  @media.delete
  asset.reload.should be_nil
end

Ahh right of course. Cool thanks I've got it now.