bar.php

<?php

class Bar extends Eloquent {

   public function foo() {
       return $this->belongsTo('Foo');
   }

}

events.php

<?php

Bar::saving(function($bar) {
    if ( $bar->isDirty('foo_id') ) {
        // relation being created or updated
    }
}

foo.php

<?php

class Foo extends Eloquent {

   public function bars() {
       return $this->hasMany('Bar');
   }

}