title: how to return choices value in mongoengine stringfield
body: <p>Hi i'm using flask with mongoengine here is my model</p>
  
  <pre><code>class Change(db.Document):
      code = (('CAD', 'Dolar Canadiense'),
              ('GBP', 'Libra Esterlina'),
              ('MXN', 'Peso Mexicano'),
              ('CHF', 'Franco Suiso'),
              ('EUR', 'Euro'),
              ('DKK', 'Corona Danesa'),
              ('SEK', 'Corona Sueka'),
              ('JPY', 'Yen Japones'),
              ('CUP', 'Peso Cubano'),
              ('PAB', 'Balboa Panama'),
              ('AUD', 'Dolar Australiano'),
              ('USD', 'Dolar Estadounidense'),
              ('NOK', 'Corona Noruega'))
  
      fecha = db.DateTimeField(required = True, unique_with = 'moneda')
      moneda = db.StringField(max_length=3, choices=code, required = True)
      recanje = db.FloatField(required  = True)
      canje = db.FloatField(required = True)
  
      @db.queryset_manager
      def objects(doc_cls, queryset):
          return queryset.order_by('date')
  </code></pre>
  
  <p>How can i get the values of de choices when i make the query:</p>
  
  <pre><code>moneda = Exchange.objects.distinct("moneda")
  </code></pre>
  
  <p>return:</p>
  
  <pre><code>['Dolar Canadiense','Libra Esterlina'...]
  </code></pre>