GeoDjango: Distance Object is not serializable

Question

I am just learning the geo-django. I can find out the distance of all places from a point. But when I use .values method to the annotated distance field, I am getting

TypeError: Object of type 'Distance' is not JSON serializable

Here is my code snippets

#models.py
import uuid
from django.contrib.gis.db import models
from django.contrib.gis.db.models.functions import Distance
from django.contrib.gis.geos import Point
class PlaceManager(models.GeoManager):
    def get_queryset(self):
        qs = super(PlaceManager, self).get_queryset()
        qs = qs.annotate(
        latitude=models.ExpressionWrapper(models.Func('position', function='ST_X'), output_field=models.FloatField()),
        longitude=models.ExpressionWrapper(models.Func('position', function='ST_Y'), output_field=models.FloatField()),
    )
    return qs.distinct()

    def nearby_places(self, lat, lng):
        p = Point(lat, lng, srid=4326)
        qs = self.get_queryset()
        qs = qs.annotate(
            distance=Distance('position', p)
        )
        return qs.order_by('distance').distinct()


class Place(models.Model):
    id = models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, db_index=True)
    position = models.PointField()
    address = models.TextField(default=None, null=True, blank=True)
    objects = PlaceManager()

    def __str__(self):
        return '{},{}'.format(self.position.x, self.position.y)

Now the code snippets I have is like this

from rest_framework.views import APIView
from rest_framework import status
from rest_framework.response import Response

class NearbyPlaces(APIView):
    def get(self, request):
        p = Place.objects.nearby_places(30.45, -90.43)
        p = p.values('distance', 'address', 'latitude', 'longitude')
        return Response(p, status=status.HTTP_200_OK)

The value of p here is like this

<GeoQuerySet [{'distance': Distance(m=7596021.71574835), 'address': 'New York City, New York','latitude': 13.4586, 'longitude': 45.6789}]>

So all I needed here is 'distance': 7596021.71574835 instead of 'distance': Distance(m=7596021.71574835)

Any help on this? Thanks in advance.

Maclean

I would suggest not using .values(), but instead use a serialiser. This way, when the serialiser gets the queryset, you can pick out the distance value and set it as the value. django-rest-framework.org/tutorial/1-serialization/… – Scutellation

Well found another way to do it. Just made an renderer and used it as default renderer class in django rest framework. – Maclean

In your #models.py, just change distance=Distance('position', p) to distance=Distance('position', p).m and that will work. This way, you can keep all of the other code as it was originally, without the need for your Renderer class. – Scutellation

@ErionV here the value p may be dynamic. – Maclean

Answer 1

there is a more simple way, only need to change content in serializer.py:

class CitySerializer(serializers.ModelSerializer):

    distance = serializers.SerializerMethodField()

    def get_distance(self, obj):

        return obj.distance.m

    class Meta:
        model = City
        fields = ('distance')

Schaefer

This is much better solution, the first solution is an overkill all the way – Alisiaalison

Answer 2

Just Found a way to do it. Had to make a Renderer class in rest framework and handled the Distance objects in there. Code snippets are like this.

encoders.py

from rest_framework.utils.encoders import JSONEncoder
from django.contrib.gis.measure import Distance


class CustomJsonEncoderWithDistance(JSONEncoder):
    def default(self, obj):
        if isinstance(obj, Distance):
            print(obj)
            return obj.m
        return super(CustomJsonEncoderWithDistance, self).default(obj)

renders.py

from rest_framework.renderers import JSONRenderer
from .encoders import CustomJsonEncoderWithDistance


class CustomJsonRenderer(JSONRenderer):
     encoder_class = CustomJsonEncoderWithDistance

settings.py

...
REST_FRAMEWORK = {
    'DEFAULT_RENDERER_CLASSES': (
        'app.renderers.CustomJsonRenderer',
    )
}

Maclean