Two has-and-belongs-to-many Properties to the same model

gistfile1.rb

class Comparison
  include DataMapper::Resource
  property :id, Serial
  has n, :msruns, :through => Resource
  has n, :msruns, :through => Resource

  belongs_to :metric
end

class Msrun
  include DataMapper::Resource
  property :id, Serial, :key => true
  has n, :comparisons, :through => Resource
end