scope.js

DS.Model.reopenClass({
  scope: function(name, fn) {
    Ember.defineProperty(this, '_'+name+'Filter', Ember.computed(function() {
      return this.filter(fn);
    }));
    
    Ember.defineProperty(this, name, Ember.computed('_'+name+'Filter.@each.id', function() {
      return this.get('_'+name+'Filter')
    }));

    this[name] = function() {
      return Ember.get(this, '_'+name);
    };
  }
});

// Long hand

App.Page = Ember.Object.extend({
  contacts: (function() {
    return this.get('contactsFilter');
  }).property('contactsFilter.@each.id')
  
  contactsFilter: (function() {
    return Contact.filter(function() {
      // your logic
    });
  }).property()
});