Django REST Framework: how to substitute null with empty string?

You could use a SerializerMethodField.

class AttendanceSerializer(serializers.ModelSerializer):

    face_image = serializers.SerializerMethodField()

    class Meta: 
        model = Attendance
        fields = ('id', 'face_image')

    def get_face_image(self, instance):
        return (instance.face_image.url if instance.face_image else '')

Override the to_representation() method of AttendanceSerializer as,

class AttendanceSerializer(serializers.ModelSerializer):
    class Meta:
        model = Attendance
        fields = ('id', 'face_image')

    def to_representation(self, instance):
        data = super().to_representation(instance)
        if not data['face_image']:
            data['face_image'] = ""
        return data

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UPDATE (11/03/2019)

I have lots of fields and its not really good idea that I repeat this code for all of them, isn't there a better way to do it for all fields?

class AttendanceSerializer(serializers.ModelSerializer):
    class Meta:
        model = Attendance
        fields = ('id', 'face_image')

    def to_representation(self, instance):
        my_fields = {'field_1', 'field_2', 'field_3', 'field_4', 'field_5'}
        data = super().to_representation(instance)
        for field in my_fields:
            try:
                if not data[field]:
                    data[field] = ""
            except KeyError:
                pass
        return data

Code for iteration over all fields mentioned in the Serializer Meta:

def to_representation(self, instance):
    data = super().to_representation(instance)
    for key, value in data.items():
        try:
            if not value:
                data[key] = ""
        except KeyError:
            pass
    return data