payment and user.rb

class Payment
  include DataMapper::Resource

  property :id, Serial

  belongs_to :payee, 'User'
  belongs_to :payer, 'User'
end

class User
	include DataMapper::Resource
	has n, :payments_as_payer, Payment, :child_key => [:payer_id]
	has n, :payments_as_payee, Payment, :child_key => [:payee_id]
	has n, :payees, User, :through => :payments_as_payer, :via => :payee_id
	has n, :payers, User, :through => :payments_as_payee, :via => :payer_id
end