Should I should subclass this Rails model?

Question

I have a model called Coupon, which can either be set to have a money_off or percent_off attributes (it can only have one set at a time).

Also depending on whether the Coupon is money_off or percent_off changes which methods are used.

Im wondering if i should be using Single table inheritance to eseentially sub class Coupon and have a sub class that deals with percent off coupons and another dealing with money off coupons?

I would then like to know how a user would be able to select this from the view.

Answer 1

The best way is to determine which functionality you require for each class. If you only need a small amount of changes, then stick to a single class with an enum:

#app/models/coupon.rb
class Coupon < ActiveRecord::Base
   enum type: [:percent, :money]

   def value
      if type.percent?
         # ...
      elsif type.money?
         # ...
      end
   end
end

This will allow you to use the type in your instance methods, which shouldn't cause such a problem if you didn't have a lot of changes to make within the class.

This would allow you to call:

@coupon = Coupon.find x
@coupon.value #-> returns value based on the type

--

The alternative (STI) would be more of a structured change, and would only work if you were referencing each class explicitly:

#app/models/coupon.rb
class Coupon < ActiveRecord::Base
end

#app/models/percent.rb
class Percent < Coupon
   def amount
      # ...
   end
end

#app/models/money.rb
class Money < Coupon
   def takeout
      # ...
   end
end

An important factor here is how you call these.

For the above classes, you have to reference the subclassed classes on their own:

@percentage_coupon = Percent.find x
@money_coupon      = Money.find y

This will obviously be more cumbersome, and may even cause problems with your routes & controllers etc.

.... so it may be best going with the single class :)