models.rb

class Mailbox
  include DataMapper::Resource
  
  property :id, Serial
  property :title, String, :required => true, :length => 1..30
  property :message, Text, :required => true, :length => 1..3000
  property :date_sent, DateTime, :default => proc { DateTime.now }

  belongs_to :to, 'Player'
  belongs_to :from, 'Player'
end

class Player
[snip]

  has n, :inbox, 'Mailbox', :child_key => :to_id
  has n, :outbox, 'Mailbox', :child_key => :from_id
end