Question

I would like to expand the functionality of some class using class_eval. I would like to force the class to inherit some methods from some other class.

I.e.:

SomeClass.class_eval do
  # force inheritence from some other class
end

What’s the best way to achieve it?

Answer 1

If overriding existing functionality is a hard requirement here, you need to have those existing methods defined in a module that’s also included.

class SomeClass
  include DefaultBehaviour
end

module DefaultBehaviour
  def run
    puts "ran default"
  end
end

module AlternateBehaviour
  def run
    puts "ran alternate"
  end
end

SomeClass.class_eval {
  include AlternateBehaviour
}

SomeClass.new.run #=> "ran alternate"

The reason for this is because of ruby’s method lookup path.

It starts off as SomeClass -> Object.

When you include AlternateBehaviour, it becomes SomeClass -> AlternateBehaviour -> Object. So methods defined directly on SomeClass still take precedence.

However, if those methods are defined on DefaultBehaviour, the lookup path becomes SomeClass -> AlternateBehaviour -> DefaultBehaviour -> Object, so your alternate method takes priority. Whichever module was included most recently is the highest priority.

In the case where you do not have control of the original class, you can do instead:

module AlternateBehaviour
  def self.included(base)
    base.send(:remove_method, :run)
  end

  def run
    puts "ran alternate"
  end
end

Though at this point, one starts to wonder whether you might be better off by just doing

SomeClass.class_eval {
  def run
    "ran alternate"
  end
end