Ruby class instance variables and inheritance

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 lik

Answer 1

To expand on @Nick Vanderbilt's answer, using active_support you do this, which is exactly the short hand I want for this functionality. Here's a complete example:

require 'active_support/core_ext'

class Foo
  class_attribute :attributes
  self.attributes = ['title','authors','location']
end

class Bar < Foo
  self.attributes = Foo.attributes + ['ISBN', 'pages']
end

puts Foo.attributes.inspect #=> ["title", "authors", "location"]
puts Bar.attributes.inspect #=> ["title", "authors", "location", "ISBN", "pages"]

Shame it's so difficult for ruby to achieve this without needing a library for it. It's the only thing I miss from python. And in my case, I don't mind the dependency on the active_support gem.

Answer 2

This is for strings (anything really), rather than arrays, but...

class A
  def self.a
    @a || superclass.a rescue nil
  end

  def self.a=(value)
    @a = value
  end

  self.a = %w( apple banana chimp )
end

class B < A
end

class C < B
  self.a += %w( dromedary elephant )
end

class D < A
  self.a = %w( pi e golden_ratio )
end



irb(main):001:0> require 'test2'
=> true
irb(main):002:0> A.a
=> ["apple", "banana", "chimp"]
irb(main):003:0> B.a
=> ["apple", "banana", "chimp"]
irb(main):004:0> C.a
=> ["apple", "banana", "chimp", "dromedary", "elephant"]
irb(main):005:0> D.a
=> ["pi", "e", "golden_ratio"]
irb(main):006:0> A.a = %w( 7 ) 
=> ["7"]
irb(main):007:0> A.a
=> ["7"]
irb(main):008:0> B.a
=> ["7"]
irb(main):009:0> C.a = nil
=> nil
irb(main):010:0> C.a
=> ["7"]

Answer 3

Another solution would be to use the inherited hook:

class LibraryItem < Object
  class << self
    attr_accessor :attributes
    def inherit_attributes(attrs)
      @attributes ||= []
      @attributes.concat attrs
    end

    def inherited(sublass)
      sublass.inherit_attributes(@attributes)
    end
  end
  @attributes = ['title', 'authors', 'location',]
end

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

Answer 4

ActiveSupport has class_attribute method in rails edge.

Answer 5

In LibraryBook variable @attributes is a new independent variable, instance variable of object LibraryBook, so its not initialized and you get error "undefined method ... for nil"
You should to initialize it by LibraryItem attribut's list before using

class LibraryBook < LibraryItem
  @attributes = LibraryItem::attributes + ['ISBN', 'pages']
end

Answer 6

You can do it using CONSTANTS also. No check though.

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

class LibraryBook < LibraryItem
  ATTRIBUTES .push('ISBN', 'pages']
end