abstract_class.rb

module AbstractClass
  def self.extended (base)
    base.instance_eval do
      self.abstract_class = true if respond_to?(:abstract_class=)
      @abstract_class = true unless instance_variable_defined?(:@abstract_class)
    end
  end

  def new (*)
    if @abstract_class
      raise NotImplementedError,
            "#{self} is an abstract class and cannot be instantiated."
    end
    super
  end

  def inherited (subclass = nil, &block)
    if subclass.nil?
      (@_inherited_blocks ||= []) << block
    else
      super
      if instance_variable_defined?(:@_inherited_blocks)
        @_inherited_blocks.each { |inherit| subclass.instance_eval(&inherit) }
      end
    end
  end
end

company.rb

class Company < Organization
  belongs_to :type,
             :class_name => 'CompanyType',
             :inverse_of => :companies
  belongs_to :size,
             :class_name => 'CompanySize',
             :inverse_of => :companies
  #...snip...
end

organization.rb

class Organization < ActiveRecord::Base
  extend AbstractClass

  inherited do
    # ...snip...
    self.inheritance_column = nil

    scope :default, -> { exists.shown }

    has_many :users,
             -> { exists }

    validates :type,
              :presence => true
    validates :size,
              :allow_nil => true,
              :presence  => true
  end
end