Question

If I have a model like this:

class Media
  attr_reader :title, :created_at
end

class Video < Media
end

class Picture < Media
  attr_reader :image_url
end

So, in the Picture class there is one attribute that should not exist in the superclass. What people usually do when you want to retrieve all the media? Would you want to throw exception on the method image_url? Or how would you re-model it?

Answer 1

If you’re interested in all media, then you shouldn’t care if it’s a video or a picture. In other words, a method that accepts a collection of Media should not call any methods from Media descendants. If you need all pictures, then work with Picture, don’t overgeneralize.

If you still do need to process all media while telling the types apart, you can branch on a type.

def process_media media
  media.each do |m|
    if m.is_a? Picture
      # work with picture
    elsif m.is_a? Video
      # work with video
    end
  end
end

An alternative is to use duck typing. That is, if it has a method image_url, then it must be some kind of a picture.

def process_media media
  media.each do |m|
    if m.respond_to? :image_url
      # work with picture
    elsif m.respond_to? :bitrate
      # work with video
    end
  end
end