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, :secondary_msruns, "Msrun", :through => SecondaryMsrunComparison, :via => :msrun 

  belongs_to :metric
end

class Msrun
  include DataMapper::Resource
  property :id, Serial, :key => true
  has n, :comparisons, :through => Resource
  has n, :secondary_comparisons, "Comparison", :through => SecondaryMsrunComparison, :via => :comparison
end

class SecondaryMsrunComparison
  include DataMapper::Resource
  belongs_to :msrun, :key => true
  belongs_to :comparison, :key => true
end