# app/models/post.rb
class Post < ActiveRecord::Base
  # ASSOCIATIONS
  belongs_to :postable, :polymorphic => true, :counter_cache => true
end

# app/models/destination.rb
class Destination < ActiveRecord::Base
  # ASSOCIATIONS
  has_many :posts, :as => :postable, :dependent => :delete_all
end

# migration
class CreatePosts < ActiveRecord::Migration
  def self.up
    create_table :posts do |t|
      t.string :title
      t.string :permalink
      t.text :content
      t.datetime :activated_at
      t.references :postable, :polymorphic => true # campo polimórfico
      t.references :user

      t.timestamps
    end
    add_index :posts, :permalink
  end

  def self.down
    remove_index :posts, :permalink
    drop_table :posts
  end
end