Question

I have a Ruby class called LibraryItem. I want to associate with every instance of this class an array of attributes. This array is long and looks something like

['title', 'authors', 'location', ...]

Note that these attributes are not really supposed to be methods, just a list of attributes that a LibraryItem has.

Next, I want to make a subclass of LibraryItem called LibraryBook that has an array of attributes that includes all the attributes of LibraryItem but will also include many more.

Eventually I will want several subclasses of LibraryItem each with their own version of the array @attributes but each adding on to LibraryItem‘s @attributes (e.g., LibraryBook, LibraryDVD, LibraryMap, etc.).

So, here is my attempt:

class LibraryItem < Object
  class << self; attr_accessor :attributes; end
  @attributes = ['title', 'authors', 'location',]
end

class LibraryBook < LibraryItem
  @attributes.push('ISBN', 'pages')
end

This does not work. I get the error

undefined method `push' for nil:NilClass

If it were to work, I would want something like this

puts LibraryItem.attributes 
puts LibraryBook.attributes

to output

['title', 'authors', 'location']
['title', 'authors', 'location', 'ISBN', 'pages']

(Added 02-May-2010)
One solution to this is to make @attributes a simple instance variable and then add the new attributes for LibraryBoot in the initialize method (this was suggested by demas in one of the answers).

While this would certainly work (and is, in fact, what I have been doing all along), I am not happy with this as it is sub-optimal: why should these unchanging arrays be constructed every time an object is created?

What I really want is to have class variables that can inherit from a parent class but when changed in the child class do not change in the the parent class.

Answer 1

Since you mention that the attributes are “fixed” and “unchanging”, I am assuming that you mean that you will never change their value once the object is created. In that case, something like the following should work:

class Foo
    ATTRS = ['title', 'authors', 'location']
    def attributes
        ATTRS
    end
end

class Bar < Foo
    ATTRS = ['ISBN', 'pages']
    def attributes
        super + ATTRS
    end
end

You are manually implementing a reader method (instead of letting attr_accessor create it for you) that disguises the internal name of the array. In your subclass, you simply call the ancestor class’ reader function, tack on the additional fields associated with the child class, and return that to the caller. To the user, this appears like a read-only member variable named attributes that has additional values in the sub-class.