How to apply a transformation on a field when serializing?

Question

In Django, how does one apply a custom transformation while serializing a field?

For instance, I have a model which has a geometry field, which is stored in a specific coordinate system. Now, for this one serializer, I'd like to perform a conversion that converts the coordinates to another coordinate system. How is that done?

The serializer currently looks like this:

class LinkWithGeometrySerializer(serializers.ModelSerializer):
    class Meta:
        model = Link
        fields = ['link_type',
                  'geometry',
                  ]

The geometry is the field that should have a transformation applied to it.

Answer 1

As Iklinac pointed out, you can use a custom field, but that only pays off when you can reuse it.

There's two other common approaches:

Keep it at serializer level:

class LinkWithGeometrySerializer(serializers.ModelSerializer):
    geometry = serializers.SerializerMethodField()
    class Meta:
        model = Link
        fields = ['link_type', 'geometry',]

    @staticmethod
    def get_geometry(obj: Link):
        # for example obtain srid from context, by passing it in via view or hardcode
        return obj.geometry.transform(srid=your_srid)

At the model level (make the database do the transformation):

in your view:

from django.contrib.gis.db.models.functions import Transform
TARGET_SRID = 4326
class LinkView(RetrieveAPIVIew):
    queryset = Link.objects.annotate(transformed=Transform("geometry", TARGET_SRID))
    ...

or (srid passed as path component in url):

from django.contrib.gis.db.models.functions import Transform
class LinkView(RetrieveAPIVIew):
    def get_queryset(self):
        return Link.objects.annotate(transformed=Transform("geometry", self.kwargs["srid"])

serializer

class LinkWithGeometrySerializer(serializers.ModelSerializer):
    geometry = serializers.GeometryField(source='transformed') # [1]
    class Meta:
        model = Link
        fields = ['link_type', 'geometry',]

[1] https://github.com/openwisp/django-rest-framework-gis/blob/master/rest_framework_gis/fields.py#L13