Django Rest Framework: MultipleObjectsReturned

Question

It´s a requirement of the project to start from an existing database. In which there are tables with compound keys. When generating the models from this database, a field was left as the primary key, and a constraint unique with both fields that previously formed the composite key.

An example follows:

models.py

class Preciosventalista(models.Model):
    idlistaprecio = models.OneToOneField(Listasprecios, models.DO_NOTHING, db_column='idlistaprecio', primary_key=True)
    idarticu = models.ForeignKey(Articulos, models.DO_NOTHING, db_column='idarticu')
    porcendescue = models.DecimalField(max_digits=6, decimal_places=2)
    cohefi = models.DecimalField(max_digits=10, decimal_places=4)
    precioneto = models.DecimalField(max_digits=10, decimal_places=4)
    precioventa = models.DecimalField(max_digits=10, decimal_places=2)

    class Meta:
        managed = True
        db_table = 'preciosventalista'
        unique_together = (('idlistaprecio', 'idarticu'),)

serializers.py


class ArticulosSerializer(serializers.ModelSerializer):
    class Meta:
        model = Articulos
        fields='__all__'        

class PreciosVentaListaSerializer(serializers.ModelSerializer):
    articulo = ArticulosSerializer(source='idarticu', read_only=True)
    class Meta:
        model = Preciosventalista
        fields='__all__'

apiviews.py


class PreciosVentaListaList(generics.ListCreateAPIView):
    queryset = Preciosventalista.objects.all()
    serializer_class = PreciosVentaListaSerializer

class PreciosVentaListaDetalle(generics.RetrieveDestroyAPIView):
    queryset = Preciosventalista.objects.all()
    serializer_class = PreciosVentaListaSerializer

urls.py


urlpatterns = [ 
    path('v1/preciosventalista/', PreciosVentaListaList.as_view(), name='preciosventalista_list'),
    path('v1/preciosventalista/<int:pk>', PreciosVentaListaDetalle.as_view(), name='preciosventalista_detalle')
]

The error when calling the service ( /api/v1/preciosventalista/1 ) from postman is:

MultipleObjectsReturned at /api/v1/preciosventalista/1 get() returned more than one Preciosventalista -- it returned 75!

I couldn´t get my service to return a set of instances or to be able to filter through both fields (idlistaprice, idarticu) to return a single instance. I have tried the following without results:

apiviews.py


class PreciosVentaListaDetalle(generics.RetrieveDestroyAPIView):
    queryset = Preciosventalista.objects.all()
    serializer_class = PreciosVentaListaSerializer

    def list(self, request):
        queryset = self.get_queryset()
        serializer = UserSerializer(queryset, many = true)
        return Response(serializer.data)

If I am making beginner mistakes I apologize but I have been working with the framework for a short time, I would appreciate any help or guidance that can help me solve the problem. Thank you so much!

RemcoGerlich · Accepted Answer · 2019-09-18 11:51:49Z

1

Django does not support compound keys. You can tell it one of the fields is a primary key, but that is not the reality in the database (there are 75 records with the same value for that field, so it's not a key).

So a URL of the form v1/preciosventalista/<int:pk> will never work, regardless of what backend framework you use -- one integer is simply not enough information to specify which record you want.

You could have URLs of the form v1/preciosventalista/<int:idlistaprecio>/<int:idarticu> so you know both numbers; but you cannot use the builtin Rest Framework generic view classes as, again, Django does not support primary keys. You'll have to make your own view classes.

answered Sep 18, 2019 at 11:51

RemcoGerlich

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1 Comment

María Emilia Diaz Over a year ago

Finally I pass both fields in my url and do the filter in the view. Thank you very much for your suggestion was a great help.

María Emilia Diaz · Accepted Answer · 2019-09-18 12:51:42Z

0

Finally, implement the view as follows:

apiviews.py


class PreciosVentaListaList(generics.ListCreateAPIView):
    def get_queryset(self):
        queryset = Preciosventalista.objects.filter(idarticu=self.kwargs["pk"], idlistaprecio=self.kwargs["id"])
        return queryset
    serializer_class = PreciosVentaListaSerializer

answered Sep 18, 2019 at 12:51

María Emilia Diaz

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