discourse/db/migrate/20131015131652_create_post_details.rb

class CreatePostDetails < ActiveRecord::Migration
  def change
    create_table :post_details do |t|
      t.belongs_to :post
      t.string     :key
      t.string     :value, size: 512
      t.text       :extra

      t.timestamps null: false
    end

    add_index :post_details, [:post_id, :key], unique: true
  end
end