elemental.rb

class Element
  include DataMapper::Resource
  
  property :id, Serial
  property :name, String
end

class Composition
  include DataMapper::Resource
  
  property :id, Serial
  belongs_to :primary_element_id, 'Element'
  belongs_to :secondary_element_id, 'Element'
  belongs_to :result_element_id, 'Element'
  
  def self.find_by_elements element1, element2
    all(primary_element_id: element1.id, secondary_element_id: element2.id) | 
      all(primary_element_id: element2.id, secondary_element_id: element1.id)
  end
end