Rails中的模型继承

Question

I have a class Item and another Weapon . They share a lot of the same state (eg size , value ), but Weapon has some extra stuff, like damage , critical , and threat . In raw Ruby, I'd extract these common attributes to a module and compose both of them, or I might subclass Weapon from Item and put the common state in the superclass.

Except I'm using Rails. Single-Table Inheritance doesn't seem like a nice solution as Weapon has a number of attributes not in Item , and I don't fancy all the empty DB columns. Polymorphic association doesn't right either, as it seems to be more useful for sharing associations between models, rather than sharing state. Using a Concern: much the same issue.

Is there anything in Rails that allows me to extend the state of a single model in another, while building an extra table for any additional attributes?

Here's a code example of what I'd like in raw Ruby:

class Item
  attr_accessor :value, :weight, item_type 

class Weapon < Item
  attr_accessor :damage, :threat, :critical, :weapon_type

Weapon.new(value: 12) # => weapon instance with value 12

In Rails, can try adding item_id to Weapon - but then I can't do Weapon.new(value: 12) , I'd have to do Weapon.new(item: Item.new(value: 12)) .

Answer 1

TL;DR this violates the Liskov Substitution Principle , so I shouldn't do it.

According to Liskov, object subtypes should be interchangeable - that is, a method that uses an instance of the Weapon class should work if I switch the Weapon for a Potion or a Gem . Clearly, that's not the case: so from an OOP angle, I shouldn't be using Inheritance here.

Thanks for the debate!