Changing a class of an object in ActiveRecord

Question

Let's say I have a FireNinja < Ninja object in my database, stored using single table inheritance. Later, I realize he's really a WaterNinja < Ninja. What's the cleanest way to change him to the different subclass? Even better, I'd love to create a new WaterNinja object and just replace the old FireNinja in the DB, preserving the ID.

Edit I know how to create the new WaterNinja object from my existing FireNinja, and I also know I can delete the old one and save the new one. What I'd like to do is mutate the class of the existing item. Whether I do that by creating a new object and doing some ActiveRecord magic to replace the row, or by doing some sort of crazy thing to the object itself, or even by deleting it and reinserting with the same ID is part of the question though.

Answer 1

You will have to define a WaterNinja#to_fireninja method. Ruby has no way to change the class of an object while keeping it the same object.

class WaterNinja < Ninja
  def to_fireninja
    FireNinja.new :name => name
  end
end

Answer 2

You need to do two things:

• Create WaterNinja < Ninja
• In ninjas table run something like UPDATE ninjas SET (type = 'WaterNinja') WHERE (type = 'FireNinja')

That's about it.

For doing runtime conversion this will do it, but I don't recommend.

class Ninja
  def become_another_ninja(new_ninja_type)
    update_attribute(:type, new_ninja_type)
    self.class.find(id)
  end
end

@water_ninja = @fire_ninja.become_another_ninja('WaterNinja')

The problem with this is that @fire_ninja will now be a throwaway object.

Answer 3

Favor composition over inheritance - you should be using the strategy pattern to change this type of behavior at runtime.

http://en.wikipedia.org/wiki/Strategy_pattern