Serializing data in Django Rest Framework for custom fields

[arjun....@zapprx.com]

Had a certain query regarding a certain implementation.Please do let me know if this is not the right forum...

My models.py is as follows:

class Prescription(models.Model):
    pr_id = models.CharField()
    date_prescribed = models.DateTimeField()
    doctor = models.ForeignKey('Doctor')
    pharmacy = models.ForeignKey('Pharmacy')  

class Doctor(models.Model):
    name = models.CharField()
    age = models.IntegerField()

class Pharmacy(models.Model):
    name = models.CharField()
    status = models.CharField()

Now i need to get the count of all pr_id grouped by month starting from this month and going back 6 months.This data needs to be in json as it needs to be sent to Angular to render a line chart. My views.py is as follows:

class PrescriptionTrendListView(generics.ListAPIView):
    end_date = datetime.utcnow().date()
    start_date = end_date + relativedelta(months=-6)    
    queryset = (Prescription.objects.extra(select={'month': connection.ops.date_trunc_sql('month', 'date_prescribed')})
                            .filter(date_prescribed__range=(start_date,end_date))
                            .values('month')
                            .annotate(Count('pr_id'))
                            .order_by('month'))
    serializer_class = LineGraphSerializer

The queryset works because I tried it in the shell and it gives me the correct data as follows:

>>> queryset
[{'pr_id__count': 16, 'month': datetime.datetime(2015, 2, 1, 0, 0, tzinfo=<UTC>)}, 
 {'pr_id__count': 71, 'month': datetime.datetime(2015, 3, 1, 0, 0, tzinfo=<UTC>)}, 
 {'pr_id__count': 75, 'month': datetime.datetime(2015, 4, 1, 0, 0, tzinfo=<UTC>)}, 
 {'pr_id__count': 96, 'month': datetime.datetime(2015, 5, 1, 0, 0, tzinfo=<UTC>)}, 
 {'pr_id__count': 99, 'month': datetime.datetime(2015, 6, 1, 0, 0, tzinfo=<UTC>)}, 
 {'pr_id__count': 93, 'month': datetime.datetime(2015, 7, 1, 0, 0, tzinfo=<UTC>)}, 
 {'pr_id__count': 39, 'month': datetime.datetime(2015, 8, 1, 0, 0, tzinfo=<UTC>)}]

However,I am not sure how to create the LineGraphSerializer. The two fields that the queryset gives are the count of the Ids and the months which are not in the initial model.Would you have any idea as to how would I go about creating this file.Any help will be greatly appreciated.

Regards,

Arjun

[Chris Foresman]

I'd do something like:

class LineGraphSerializer(serializers.Serializer):
    count = serializers.IntegerField(source='pr_id__count')
    month = serializers.SerializerMethodField()


    def get_month(self, obj):
        return '{0}-{1}'.format(obj.month.year, obj.month.month)

You might need to play with that a bit to get it right (will obj be a dict or object, I'm not sure...), but I think that's on the right track.

[Arjun Nambiar]

Looks promising..will give that a shot..thanks..

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Regards,

Arjun

[arjun....@zapprx.com]

So this is what my final code looks like :

models.py

class Prescription(models.Model):
    date_prescribed = models.DateTimeField()
    doctor = models.ForeignKey(Doctor)
    pharmacy = models.ForeignKey(Pharmacy)  

class Doctor(models.Model):
    name = models.CharField()
    age = models.IntegerField()

class Pharmacy(models.Model):
    name = models.CharField()
    status = models.CharField()

views.py

class PrescriptionTrendListView(generics.ListAPIView):
    queryset = Prescription.objects.all()
    serializer_class = LineGraphSerializer

    def get_queryset(self):
        end_date = timezone.now()
        start_date = end_date - relativedelta(months=6)
        truncate_date = connection.ops.date_trunc_sql('month', 'date_prescribed')
        qs = super(PrescriptionTrendListView,self).get_queryset.extra(select={'month': truncate_date})
        return qs.filter(date_prescribed__range=(start_date, end_date)).annotate(pk_count=Count('pk')).order_by('month'

and my serializer.py

from rest_framework import serializers
from .models.Prescription import Prescription

class LineGraphSerializer(serializers.ModelSerializer):
    pk_count = serializers.IntegerField(read_only=True)
    month = serializers.DateTimeField(read_only=True)

    class Meta:
    	model = Prescription
    	fields = ['pk_count', 'month']

    

However this gives an error stating NoneType object is not iterable.Full traceback here .. Any help ??

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Regards,

Arjun

[Tymur Maryokhin]

Upon reading the documentation http://www.django-rest-framework.org/api-guide/serializers/#read-only-baseserializer-classes:

class LineGraphSerializer(serializers.BaseSerializer):
    def to_representation(self, obj):
        return {
            'pk_count': obj['pk_count'],
            'month': obj['month']
        }

And don't forget to define the get method in the viewset to pass the data as a list of dictionaries:

def get(self, request, *args, **kwargs):
    graph_data = self.get_queryset().values('pk_count', 'month')
    serializer = self.get_serializer(data=graph_data, many=True)
    return Response(serializer.data)

No guarantee that this 100% works, but probably closer to what you want.

понедельник, 24 августа 2015 г., 23:03:15 UTC+2 пользователь arjun....@zapprx.com написал:

[arjun....@zapprx.com]

Hi Tymur,

Thanks for that..but still no luck..get the same error..

...

[Chris Foresman]

Are you calling is_valid()? In your view, try then printing serializer.validated_data and see what gets returned. 

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[Arjun Nambiar]

Chris,

Pardon my ignorance..but how would i do that ?

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Regards,

Arjun

[Chris Foresman]

I think someone recommended the following:

```python

def get(self, request, *args, **kwargs):
    graph_data = self.get_queryset().values('pk_count', 'month')
    serializer = self.get_serializer(data=graph_data, many=True)
    return Response(serializer.data)

```

I’d recommend amending that as follows:

```python

def get(self, request, *args, **kwargs):
    graph_data = self.get_queryset().values('pk_count', 'month')

    serializer = self.get_serializer(graph_data, many=True)

    serializer.is_valid()

    print graph_data

    print serializer.validated_data
    return Response(serializer.data)

```

First, pass the results of your queryset as objects, not data. Calling is_valid() causes serialization to happen. Printing graph_data tells us what, if anything, was passed into the serializer. Printing validated_data will tell us if we were able to serialize the specific data we want from the queryset.

Chris Foresman

ch...@chrisforesman.com

[arjun....@zapprx.com]

Ok..i did that and the new error stated that LineGraphSerializer' object has no attribute 'base_fields'.

I am on DRF 2.4.6 and looks like BaseSerializer was introduced only in 3.0 :-(

Chris Foresman

ch...@chrisforesman.com

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Regards,

Arjun

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