Confused about coercions in Virtus

virtus-coercion.rb

require 'virtus'

class Point
  include Virtus
  attribute :x, Integer
  attribute :y, Integer
end

# Define the coercion
class PointAttr < Virtus::Coercion::Object
  primitive Point
  coercion_method :to_point
end

class Rectangle
  include Virtus
  attribute :top_left,     PointAttr
  attribute :bottom_right, PointAttr
end

module Virtus
  class Coercion
    class Array < Virtus::Coercion::Object
      def self.to_point(value)
        Point.new(x: value[0], y: value[1])
      end
    end
  end
end

rect = Rectangle.new(top_left: [3,5], bottom_right: [8,7])
rect.top_left                 # => 
rect.bottom_right             # => 
# ~> /home/avdi/.rvm/gems/ruby-1.9.3-p0/gems/virtus-0.4.1/lib/virtus/coercion/object.rb:102:in `method_missing': undefined method `coercion_method' for PointAttr:Class (NoMethodError)
# ~> 	from -:12:in `<class:PointAttr>'
# ~> 	from -:10:in `<main>'

@dkubb, @solnic: I wrote this for inclusion in an upcoming article, but gave up on it.

The point, if it's not clear from the code, is to have a Rectangle class which can accept its start and end points as either Point objects or as two-element arrays. Either way, they should end up as Point objects.

Two things:

1. I tried to crib from the docs and tests, but as you can see above it didn't work.
2. This seems really verbose; I was especially concerned about having to write a separate PointAttr class to define the conversion to Point. What am I missing?