code: function mapBy (dependentKey, propertyKey) {
        var callback = function(item) { return get(item, propertyKey); };
        return map(dependentKey + '.@each.' + propertyKey, callback);
      }
docs: Returns an array mapped to the specified key.
  
  ```javascript
  var Person = Ember.Object.extend({
  childAges: Ember.computed.mapBy('children', 'age')
  });
  
  var lordByron = Person.create({ children: [] });
  
  lordByron.get('childAges'); // []
  lordByron.get('children').pushObject({ name: 'Augusta Ada Byron', age: 7 });
  lordByron.get('childAges'); // [7]
  lordByron.get('children').pushObjects([{
  name: 'Allegra Byron',
  age: 5
  }, {
  name: 'Elizabeth Medora Leigh',
  age: 8
  }]);
  lordByron.get('childAges'); // [7, 5, 8]
  ```
  
  @method computed.mapBy
  @for Ember
  @param {String} dependentKey
  @param {String} propertyKey
  @return {Ember.ComputedProperty} an array mapped to the specified key