discourse/db/migrate/20121018133039_create_topic_allowed_users.rb

class CreateTopicAllowedUsers < ActiveRecord::Migration
  def change
    create_table :topic_allowed_users do |t|
      t.integer :user_id, null: false
      t.integer :topic_id, null: false
      t.timestamps null: false
    end

    add_index :topic_allowed_users, [:topic_id, :user_id], unique: true
    add_index :topic_allowed_users, [:user_id, :topic_id], unique: true
  end
end