payment and user.rb

class Payment
  include DataMapper::Resource
 	belongs_to :payee, User
	belongs_to :payer, User
	

	property :id, Serial
       # ...
end

class User
	include DataMapper::Resource
        
        has n, :payees, User, :child_key => [:payee_id]
        has n, :payers, User, :child_key => [:payer_id]

        property :id, Serial

end