Fastest/One-liner way to list attr_accessors in Ruby?

Heres an alternative using a mixin rather than inheritance:

module TrackAttributes
  def attr_readers
    self.class.instance_variable_get('@attr_readers')
  end

  def attr_writers
    self.class.instance_variable_get('@attr_writers')
  end

  def attr_accessors
    self.class.instance_variable_get('@attr_accessors')
  end

  def self.included(klass)
    klass.send :define_singleton_method, :attr_reader, ->(*params) do
      @attr_readers ||= []
      @attr_readers.concat params
      super(*params)
    end

    klass.send :define_singleton_method, :attr_writer, ->(*params) do
      @attr_writers ||= []
      @attr_writers.concat params
      super(*params)
    end

    klass.send :define_singleton_method, :attr_accessor, ->(*params) do
      @attr_accessors ||= []
      @attr_accessors.concat params
      super(*params)
    end
  end
end

class MyClass
  include TrackAttributes

  attr_accessor :id, :title, :body
end

MyClass.new.attr_accessors #=> [:id, :title, :body]

Extract the attributes in to an array, assign them to a constant, then splat them in to attr_accessor.

class SubClass < MyBaseClass
  ATTRS = [:id, :title, :body]
  attr_accessor(*ATTRS)
end

Now you can access them via the constant:

puts SubClass.ATTRS #=> [:id, :title, :body]

There is no way (one-liner or otherwise) to list all methods defined by attr_accessor and only methods defined by attr_accessor without defining your own attr_accessor.

Here's a solution that overrides attr_accessor in MyBaseClass to remember which methods have been created using attr_accessor:

class MyBaseClass
  def self.attr_accessor(*vars)
    @attributes ||= []
    @attributes.concat vars
    super(*vars)
  end

  def self.attributes
    @attributes
  end

  def attributes
    self.class.attributes
  end
end

class SubClass < MyBaseClass
  attr_accessor :id, :title, :body
end

SubClass.new.attributes.inspect #=> [:id, :title, :body]

Following up on Christian's response, but modifying to use ActiveSupport::Concern...

module TrackAttributes
  extend ActiveSupport::Concern

  included do
    define_singleton_method(:attr_reader) do |*params|
      @attr_readers ||= []
      @attr_readers.concat params
      super(*params)
    end

    define_singleton_method(:attr_writer) do |*params|
      @attr_writers ||= []
      @attr_writers.concat params
      super(*params)
    end

    define_singleton_method(:attr_accessor) do |*params|
      @attr_accessors ||= []
      @attr_accessors.concat params
      super(*params)
    end
  end

  def attr_readers
    self.class.instance_variable_get('@attr_readers')
  end

  def attr_writers
    self.class.instance_variable_get('@attr_writers')
  end

  def attr_accessors
    self.class.instance_variable_get('@attr_accessors')
  end

end