Add a non-model field on a ModelSerializer in DRF

Question

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):
      print(direction=validated_data['non_field'])

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?

Answer 1

Just an example might help you.

  class ExtensibleModelSerializerOptions(serializers.SerializerOptions):
    """
    Meta class options for ModelSerializer
    """
    def __init__(self, meta):
        super(ExtensibleModelSerializerOptions, self).__init__(meta)
        self.model = getattr(meta, 'model', None)
        self.read_only_fields = getattr(meta, 'read_only_fields', ())
        self.non_native_fields = getattr(meta, 'non_native_fields', ())


class ExtensibleModelSerializer(serializers.ModelSerializer):

    _options_class = ExtensibleModelSerializerOptions

    def restore_object(self, attrs, instance=None):
        """
        Deserialize a dictionary of attributes into an object instance.
        You should override this method to control how deserialized objects
        are instantiated.
        """
        for field in self.opts.non_native_fields:
            attrs.pop(field)

        return super(ExtensibleModelSerializer, self).restore_object(attrs, instance)

Source: https://github.com/tomchristie/django-rest-framework/issues/951

Answer 2

As mentioned there are two ways. (1) adding a model property. (2) adding a model field. I feel that adding a @property to model was explained well in this post. If you want to keep your models "lean and mean" use a Method field. Chandus answer omits some crucial points, though:

class DeliveryItemSerializer(serializers.ModelSerializer):

    product_name = serializers.SerializerMethodField(read_only=True)

    def get_product_name(self, obj):
        return obj.product.name

    class Meta:
        model = DeliveryItem
        fields = (
            (...your field names),
            'product_name',)

1. Set read_only
2. The field and the corresponding method do not have the same name. The method name defaults to get_field_name. If you use another name use the method_name=method name argument on SerializerMethodField()

Answer 3

class Foo(models.Model):
    . . .
    @property
    def my_field(self):
        return stuff
    . . .

Source:

Django REST Framework: adding additional field to ModelSerializer

Answer 4

class MySerializer(serializers.ModelSerializer):
    write_only_char_field = serializer.CharField(write_only=True)
    write_only_list_char_field = serializer.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)

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)

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

Answer 5

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

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

    def create(self, validated_data):
        print(direction=validated_data['non_field'])

http://www.django-rest-framework.org/api-guide/fields/#serializermethodfield

or go through this link