airmigo.rb

class Airmigo
  include DataMapper::Resource
  
  property :id, Serial
  
  belongs_to :from, :class_name => 'User'
  belongs_to :to, :class_name => 'User'

  before :save do
    Airmigo.first_or_create(:conditions => { :from => to, :to => from })
  end

  before :destroy do
    Airmigo.destroy(:from => to, :to => from)
  end

end

user.rb

# This is a default user class used to activate merb-auth.  Feel free to change from a User to 
# Some other class, or to remove it altogether.  If removed, merb-auth may not work by default.
#
# Don't forget that by default the salted_user mixin is used from merb-more
# You'll need to setup your db as per the salted_user mixin, and you'll need
# To use :password, and :password_confirmation when creating a user
#
# see merb/merb-auth/setup.rb to see how to disable the salted_user mixin
# 
# You will need to setup your database and create a user.
class User
  include DataMapper::Resource

  property :id,     Serial
  property :updated_at, DateTime
  property :login,  String, :nullable => false
  property :firstname, String
  property :lastname, String

  has n, :emails
  has n, :localairports
  has n, :airmigos, :as => :from, :dependent => :destroy
  has n, :airmigos_to_self, :as => :to, :dependent => :destroy # just there for referential integrity

end