I'm wondering if there is a way to force a subclass to override a method from its parent method in either Ruby or Rails (in Java you would do this with an abstract class/method).

Let's say I have the following classes:

class Pet
end

class Dog < Pet
  def collar_color
    "red"
  end
end

class Cat < Pet
end

dog = Dog.new
dog.collar_color
==> red

cat = Cat.new
cat.collar_color
==> NoMethodError

In this example I would never instantiate a Pet object, but it exists to serve as a way to collect common methods to common classes. But let's say I want to ensure that all subclasses of Pet override the collar_color method. Is there a way to do that? Could I achieve it through testing in some way? Assume I don't want a default defined in the parent class.

My real-life use case is a collection of classes that all have polymorphic ownership of another class. If I have a display page of the owned class, then one of the owner classes not having a method could leave me with a NoMethodError problem.

CodePudding user response：

Just define the default method implementation in the abstract class by raising Not implemented error or something. By doing that you also clarifies in your class design that when others / you want to inherit the Pet class they need to override collar_color method. Clarity is a good think and there is no benefit in not defining a default method in the abstract class.

Or if you want to achieve that by testing you can create a test case for Pet class that check if its descendants is defining their own collar_color method or not. I think Rails / Ruby 3.1 have .descendants methods defined or you can just google them.

# Pet_spec.rb 
describe "descendants must implement collar_color" do
  it "should not throw error" do
    descendants = Pet.descendants
    descendants.each do |descendant|
      expect { descendant.new.collar_color }.to.not raise_error
    end
  end
end

CodePudding user response：

in Ruby you can use the abstract keyword to define an abstract method in the parent class that must be implemented by any subclasses. For example:

class Pet
  abstract :collar_color
end

class Dog < Pet
  def collar_color
    "red"
  end
end

class Cat < Pet
  def collar_color
    "blue"
  end
end

Now, if you try to instantiate a Pet object, you will get an error:

pet = Pet.new

=> TypeError: Cannot instantiate abstract class Pet

However, the Dog and Cat classes can be instantiated and will return the correct result for the collar_color method:

dog = Dog.new
dog.collar_color

=> "red"
cat = Cat.new
cat.collar_color

=> "blue"

You can also use the prepend method to ensure that any subclasses implement the required method:

class Pet
  def collar_color
    raise "This method must be implemented by a subclass"
  end
end

class Dog < Pet
  prepend collar_color

  def collar_color
    "red"
  end
end

class Cat < Pet
  prepend collar_color

  def collar_color
    "blue"
  end
end

If any subclass does not implement the collar_color method, it will raise an error when the method is called:

dog = Dog.new
dog.collar_color

=> "red"
cat = Cat.new
cat.collar_color

=> "blue"
pet = Pet.new
pet.collar_color

=> RuntimeError: This method must be implemented by a subclass

You can also use testing to ensure that the required method is implemented in the subclasses. For example, using RSpec:

RSpec.describe Pet do
  describe "collar_color" do
   it "must be implemented by a subclass" do
    expect { subject.collar_color }.to raise_error(RuntimeError, "This method must be implemented by a subclass")
   end
  end
end

You can then include this test in the specs for each subclass to verify that they implement the required method.