title: Displaying the last object of nested relationship
body: <p>i'm having trouble with returning data from many to many relationship between two models. i wan't to show all <code>Currency? objects with their newest</code>Ticker<code>object related to that currency based on the</code>created_at` field. i'm wondering how i can create such an output in json?</p>
  
  <p><strong>desired output</strong></p>
  
  <pre><code>[
      {
          "id": 1,
          "name": "Bitcoin",
          "symbol": "BTC",
          "img": "/media/static/img/currencies/bitcoin_uzSOQkH.png",
          "tickers": 
              {
                  "rank": 1,
                  "price_dkk": "111239.222648",
                  "market_cap_dkk": 1861795518438,
                  "percent_change_1h": "0.03",
                  "percent_change_24h": "2.44",
                  "percent_change_7d": "46.80",
                  "created_at": "2017-12-12T20:11:49.995851Z"
              }
      }
  ]
  </code></pre>
  
  <p><strong>view</strong></p>
  
  <pre><code>class CurrencyGetView(ProtectedResourceView):
      def get(self, request):
          currencies = CurrencySerializer(Currency.objects.all(), many=True)
  
          return JsonResponse(currencies.data, safe=False)
  </code></pre>
  
  <p><strong>serializers</strong></p>
  
  <pre><code>class TickerSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Ticker
          fields = ('rank', 'price_dkk', 'market_cap_dkk', 'percent_change_1h', 'percent_change_24h', 'percent_change_7d', 'created_at', )
  
  
  class CurrencySerializer(serializers.HyperlinkedModelSerializer):
      tickers = TickerSerializer(many=True)
      class Meta:
          model = Currency
          fields = ('id', 'name','symbol', 'img', 'tickers',)
  </code></pre>
  
  <p><strong>models</strong></p>
  
  <pre><code>class Ticker(models.Model):
      rank = models.IntegerField()
      price_dkk = models.DecimalField(max_digits=20, decimal_places=6)
      market_cap_dkk = models.BigIntegerField()
      percent_change_1h = models.DecimalField(max_digits=4, decimal_places=2)
      percent_change_24h = models.DecimalField(max_digits=4, decimal_places=2)
      percent_change_7d = models.DecimalField(max_digits=4, decimal_places=2)
  
      created_at = models.DateTimeField(auto_now_add=True)
  
      class Meta:
          verbose_name_plural = _('Ticker')
  
      def __str__(self):
          return self.id
  
  class Currency(models.Model):
      symbol = models.CharField(max_length=4, default='BTC', unique=True)
      name = models.CharField(max_length=20, default='Bitcoin', unique=True)
      img = models.ImageField(upload_to = 'static/img/currencies', blank=True)
      is_active = models.BooleanField(default=False)
      tickers = models.ManyToManyField(Ticker)
  
      class Meta:
          verbose_name_plural = 'currencies'
  
      def __str__(self):
          return self.name
  </code></pre>