single table inheritance: must all classes in hierarchy have same properties?

I have the following

class Item < ActiveRecord::Base
end

class Tal开发者_运维知识库k < Item
end

with the migration

class CreateItems < ActiveRecord::Migration
  def self.up
    create_table :items do |t|
      t.string :type
      t.string :name
      t.text :description
      t.time :start_time
      t.time :duration
      t.timestamps
    end
  end

  ...
end

By default the description property will be available on the Item and Talk classes. Is there a way to restrict the property so that is only available to the Talk class?

---

class Item < ActiveRecord::Base
  def duration
    raise NoMethodError
  end

  def duration=(value)
    raise NoMethodError
  end
end

class Talk < Item
  def duration
    read_attribute(:duration)
  end

  def duration=(value)
    write_attribute(:duration, value)
  end
end

You could always do that, but it's a lot of work for nothing. What's the worst that will happen when you read duration on an Item? You'll get back nil, which will cause a crash shortly thereafter. You don't need to be so concerned about these types of issues in Ruby.

If needed, you can create a module and include the module in the two classes, for the shared behavior, and drop the STI.