collect_subclasses.rb

class BusinessEvent
  def self.inherited(clazz)
    subclasses << clazz
  end
  
  def self.subclasses
    @@subclasses ||= []
  end
end

class OrderEvent < BusinessEvent
end

class PurchaseEvent < BusinessEvent
end

class CustomerEvent < BusinessEvent
end

puts BusinessEvent.subclasses.inspect
# => [OrderEvent, PurchaseEvent, CustomerEvent]