I know, it assumes symbol attribute names only, this is for "simplicity" ~ lul

changeset.rb

class User
  def initialize(attributes = {})
    update_attributes(attributes)
  end

  def attributes
    @attributes ||= {}
  end

  def changes
    @changes ||= {}
  end

  def changed?
    changes.values.any? { |changeset| changeset.length > 1 }
  end

  def update_attributes(new_attributes)
    new_attributes.each_with_object(changes) do |(attribute, new_value), changeset|
      (changeset[attribute] ||= []) << new_value
    end
    attributes.merge!(new_attributes)
    self
  end

  def method_missing(method, *args, &block)
    if attributes.key?(method)
      attributes[method]
    elsif method =~ /(.*)=$/ && attributes.key?($1.to_sym)
      update_attributes($1.to_sym => args.first)
    elsif method =~ /(.*)_was$/ && attributes.key?($1.to_sym)
      changes[$1.to_sym][-2] if changes[$1.to_sym].length > 1
    else
      super
    end
  end

  def respond_to_missing?(method, _include_private = false)
    if attributes.key?(method)
      true
    elsif method =~ /(.*)=$/ && attributes.key?($1.to_sym)
      true
    elsif method =~ /(.*)_was$/ && attributes.key?($1.to_sym)
      true
    else
      super
    end
  end
end