gistfile1.rb

class Foo
  include DataMapper::Resource
  property :id, Serial

  has n, :mappings
  has n, :bars, :through => :mappings
end

class Bar
  include DataMapper::Resource
  property :id, Serial

  has n, :mappings
  has n, :foos, :through => :mappings
end

class Mapping
  include DataMapper::Resource

  property :foo_id,       Integer, :key => true
  property :bar_id,       Integer, :key => true
  property :blah,         String

  belongs_to :foo
  belongs_to :bar
end