election_migrations.rb

class MakeVoterPolymorphic < ActiveRecord::Migration
  def change
    add_column :voters, :voterable_id, :integer
    add_column :voters, :voterable_type, :string
  end
end

# you could also use a shortened syntax that would do the same thing
# and add these two columns to the table

class MakeVoterPolymorphic < ActiveRecord::Migration
  def change
    change_table :voters do |t|
      t.references :voterable, :polymorphic => true
    end
  end
end