Sending django queryset with annotate to my rest api

Question

i have two simple models with random data about phones.

class Phone(models.Model):
    name = models.CharField(max_length=200)
    description = models.CharField(max_length=400)
    image = models.ImageField(upload_to='phone_img/', max_length=255, null=True, blank=True )
    slug_phone = models.SlugField(blank=True)

class Price(models.Model):
    price = models.DecimalField(max_digits=6, decimal_places=2)
    date = models.DateField(null=True, blank=True) 
    phone = models.ForeignKey(Phone, on_delete=models.CASCADE)
    market = models.ForeignKey(Market, on_delete=models.CASCADE)

I am able to send simple data via data the REST_framwork to my API.

views.py

class SnippetList(generics.ListCreateAPIView):
    queryset = Price.objects.all()
    serializer_class = PriceSerializer

serializers.py

class PriceSerializer(serializers.ModelSerializer):

    class Meta:
        model = Price
        fields = ('id', 'phone', 'price', 'date', 'market', )


    phone = serializers.StringRelatedField()
    market = serializers.StringRelatedField()

Now i wanted to use the annotate() method with the Min() function in a QuerySet. I wanted to display the minimum price of each phone. So i tried something like this:

views.py

from django.db.models import Min

<....>

class SnippetList(generics.ListCreateAPIView):
    queryset = Price.objects.all()
    serializer_class = PriceSerializer
    def get_queryset(self):
        min_price = Phone.objects.annotate(Min('price__price'))

        return min_price

of course this did not work. i followed along the server errors messages and ended with this:

serializers.py

from rest_framework import serializers
from market.models import Price, Phone


class PriceSerializer(serializers.ModelSerializer):

    class Meta:
        model = Price
        fields = ('id', 'phone', 'price', 'date', 'market')

    class Meta:
        model = Phone
        fields = ('name', 'phone', 'market', )


    phone = serializers.StringRelatedField()
    market = serializers.StringRelatedField()

My API shows me now this output, it´s not the min_price value, but at least something:

{"count":7,"next":null,"previous":null,"results":[{"name":"SAMSUNG Galaxy S10"},{"name":"SAMSUNG Galaxy S10 plus"},{"name":"SAMSUNG Galaxy S10e"},{"name":"SAMSUNG Galaxy S8"},{"name":"SAMSUNG Galaxy S8 Plus"},{"name":"SAMSUNG Galaxy S9"},{"name":"SAMSUNG Galaxy S9 plus"}]}

So i asked myself where to put .annotate(Min ? Or should i created a single ListCreateAPIView with queryset = Phone.objects.all() ? Is it possible to display the data of min_price without a new field in my serializers.py?

If you try to find the min value from Price model, it won't return an expected result. Because, each row has only one price attribute, hence the Min(price) become price itself. — JPG
– JPG, Commented Apr 29, 2019 at 1:06
1. With the serializers.py above, no error showed up. Only when i delete the lines wich includes class meta: model = Phone. Then the Error tells me that there is no Phone and price field. 2. ok i understand. That is why thought to go in my view / get_queryset with the "Phone.objects". Because then you have a certain Phone model "Samsung S10" for example and different prices. I guess i try to write a "ListCreateAPIView" only for Phone Model. — black_hole_sun
– black_hole_sun, Commented Apr 29, 2019 at 1:19
Yeah... But you should use the Phone Model with the serializer which you are intended to serialize them (phone querysets with Phone serializer). Also, should use the Phone Api along with them — JPG
– JPG, Commented Apr 29, 2019 at 1:39

JPG · Accepted Answer · 2019-04-29 04:01:33Z

I've already mentioned some points in comments. But, I'm going to repeat that here :(

1. If you try to find the min value from Price model, it won't return an expected result. Because, each row has only one price attribute, hence the Min(price) become price itself.

2. If you want to serialize a queryset of a particular model (say Price), you should have used the ModelSerializer defined with model=Price in metaclass of the serializer.

3. You can't use two Meta classes in one serializer.

4. If you wish to display the Min(price) to the api, it is most suited to the Phone API

So,

1. Remove the `Phone` meta class from `PriceSerializer`

class PriceSerializer(serializers.ModelSerializer):
    class Meta:
        model = Price
        fields = ('id', 'phone', 'price', 'date', 'market')

    phone = serializers.StringRelatedField()
    market = serializers.StringRelatedField()

I'm not sure you are aware of the StringRelatedField() mechanism since you didn't do anything to your __str__() method of the Price model.

2. Create a new serializer class for serializing the `Price` queryset

class PhoneSerializer(serializers.ModelSerializer):
    min_price = serializers.IntegerField()
    class Meta:
        model = Phone
        fields = '__all__'

It's important to add a new field min_price to catch the annotated Min() value from the queryset.

3. Create a new view class to display the `Phone` list

class PhoneAPI(generics.ListAPIView):
    queryset = Phone.objects.annotate(min_price=Min('price__price'))
    serializer_class = PhoneSerializer

The queryset attribute must be the annotated queryset.

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Sending django queryset with annotate to my rest api

1 Answer 1

1. Remove the `Phone` meta class from `PriceSerializer`

2. Create a new serializer class for serializing the `Price` queryset

3. Create a new view class to display the `Phone` list

4. wire up the view in `urls.py`

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Collectives™ on Stack Overflow

1 Answer 1

1. Remove the Phone meta class from PriceSerializer

2. Create a new serializer class for serializing the Price queryset

3. Create a new view class to display the Phone list

4. wire up the view in urls.py

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1. Remove the `Phone` meta class from `PriceSerializer`

2. Create a new serializer class for serializing the `Price` queryset

3. Create a new view class to display the `Phone` list

4. wire up the view in `urls.py`