Python – Add a non-model field on a ModelSerializer in DRF 3


How do add a non-model field on a ModelSerializer in DRF 3? i.e. add a field that does not exist on my actual model?

class TestSerializer(serializers.ModelSerializer):
    url = serializers.HyperlinkedIdentityField(view_name='vote_detail')
    non_field = serializers.CharField()  # no corresponding model property.

    class Meta:
        model = vote_model
        fields = ("url", "non_field")

    def create(self, validated_data):

But DRF 3 gives me the error:

Got AttributeError when attempting to get a value for field `non_field` on serializer `TestSerializer`.
The serializer field might be named incorrectly and not match any attribute or key on the `Test` instance.
Original exception text was: 'Test' object has no attribute 'non_field'.

I have searched stack DRF – ModelSerializer with a non-model write_only field and found a few solutions but these refer to DRF 2 where I'm using DRF 3. Is there a solution for this on this version?

Best Solution

class MySerializer(serializers.ModelSerializer):
    write_only_char_field = serializers.CharField(write_only=True)
    write_only_list_char_field = serializers.ListField(child=serializers.CharField(max_length=100, default=''), write_only=True)
    empty_method_field = serializers.SerializerMethodField()
    read_only_custom_model_field = serializers.CharField(source='custom_property', read_only=True)

    def create(self, validated_data):
        validated_data.pop('write_only_char_field', None)
        validated_data.pop('write_only_list_char_field', None)
        return super().create(validated_data)

The serializers.CharField(write_only=True) and serializers.ListField(...) is a good solution to provide extra data to your .create() and .update() methods, as either a single string or a list of strings (you can mix ListField with other serializer field types).
With this method, you can also define def validate_write_only_char_field to implement some quick and simple validation.

serializers.SerializerMethodField() allows you to add some custom read-only field to your serializer output from a method defined on the serializer.

The read_only_custom_model_field would use a method on your model to read some data, not strictly a model field, but a custom method. I.e.

class MyModel(models.Model):
    my_field = models.CharField(max_length=100)

    def custom_property(self):
        return "Perform calculations, combine with related models, etc. etc."
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