CODEWITHSUNDEEP
pythondjangodjango-rest-framework
Peter Pik
Peter Pik

Reputation: 11193

Displaying the last object of nested relationship

i'm having trouble with returning data from many to many relationship between two models. i wan't to show all Currency? objects with their newestTickerobject related to that currency based on thecreated_at` field. i'm wondering how i can create such an output in json?

desired output

[
    {
        "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"
            }
    }
]

view

class CurrencyGetView(ProtectedResourceView):
    def get(self, request):
        currencies = CurrencySerializer(Currency.objects.all(), many=True)

        return JsonResponse(currencies.data, safe=False)

serializers

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',)

models

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

Upvotes: 3

Views: 215

Answers (2)

masterfloda
masterfloda

Reputation: 3038

I feel that this should be solved on the query level, ideally with prefetch_related. But Django can't slice Prefetch objects. Bummer. And Subquery is painstakingly slow.

The best solution I can think of, is creating a SerializerMethodField, which calls the TickerSerializer with a sliced dataset. The drawback is that DRF will fire an additional SQL query for each Currency object in the list.

from rest_framework.fields import SerializerMethodField

class CurrencySerializer(serializers.HyperlinkedModelSerializer):
    ticker = SerializerMethodField()

    def get_ticker(self, obj):
        return TickerSerializer(
            instance=obj.tickers.order_by('-created_at')[:1],
            many=True
        ).data

    class Meta:
        model = Currency
        fields = ('id', 'name','symbol', 'img', 'tickers', 'ticker')

Upvotes: 3

Pieter Hamman
Pieter Hamman

Reputation: 1612

Think something like this would work, the problem with the current implementation is that the many=True would get all the related tickers and return those objects in a list, which is the correct behaviour, but since you want to only display the latest ticker for each currency you'll need to serialize only the latest object.

class CurrencySerializer(serializers.HyperlinkedModelSerializer):
    tickers = serializers.SerializerMethodField()

    class Meta:
        model = Currency
        fields = ('id', 'name','symbol', 'img', 'tickers')

    def get_tickers(self, obj):
        data = {}
        try:
            latest_ticker = obj.tickers.latest('-created_at')
            data = TickerSerializer(instance=latest_ticker).data
        except Ticker.DoesNotExist:
            pass
        return data

Upvotes: 1

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