gistfile1.rbx

# With callback methods, if they return false, it will halt the chain and your model won't be saved.
# Ninety-nine out of a hundred times, this is only an issue if you're 'caching' a boolean value based on 
#   an association status or other convoluted evaluation

class Entity < ActiveRecord::Base
  named_scope :publicly_available, :conditions => {:publicly_available => true}

  has_many :items
  belongs_to :user

  before_save :calculate_publicly_available
  attr_protected :publicly_available

  protected

  def calculate_publicly_available
    self.publicly_available = self.items.any? && !self.user_id.nil?
    # return true here or the callback chain will halt if the statement evaluates to false
    true
  end
end

class Item < ActiveRecord::Base
  belongs_to :entity
  validates_presence_of :entity_id
end

class ItemObserver < ActiveRecord::Observer
  def after_create(item)
    save_entity(item.entity)
  end

  def after_destroy(item)
    save_entity(item.entity)
  end
  
  private
  
  def save_entity(entity)
    entity.save!
  end
end