class Organization
  belongs_to :location
  has_and_belongs_to_many :events
end

class Location
  belongs_to :city
  has_many :organizations
end

class Event
  belongs_to :city
  has_and_belongs_to_many :organizations
end

class City
  has_many :locations
  has_many :events

  after_save :set_org_ids

  def set_org_ids
    from_location = Organization.joins(:location).where(:locations => {:city_id => id})
    from_events = Organization.joins(:events).where(:events => {:city_id => id})
    # you must create an org_ids column on cities table before this will work
     self.org_ids = (from_location | from_events).collect(&:id).join(',')
     self.save
  end

  def organizations
    Organization.find(self.org_ids)
  end

end