Django-rest-framework: Serializer mixin fields not getting picked up

[vesterbaek]

I would like to declare som common fields in a mixin class, used by subclasses of both serializer.Serializer and serializer.ModelSerializer. These fields, however, seems not to be picked up, even though defined in Meta.fields. Is this by design?

Repro:

class TestSerializerMixin:
    def test_mixin(self):
        class MyMixin(object):
            some_field = serializers.BooleanField()

        class MySerializer(MyMixin, serializers.Serializer):
            class Meta:
                fields = ['some_field']

        serializer = MySerializer(data={})
        serializer.is_valid()
        assert 'some_field' in serializer.errors

=>

    assert 'some_field' in serializer.errors
E   assert 'some_field' in {}
E    +  where {} = MySerializer(data={}):.errors

[xordoquy]

Fields are gathered on classes that inherit from serializers.
Your mixin has object as parent and thus break the field introspection.

[vesterbaek]

Ok, see _declared_fields is populated in SerializerMetaclass. Would it be ok to use this class on the mixin to get the fields registered?

[xordoquy]

Should likely do the trick

[vesterbaek]

It does:

class TestSerializerMixin:
    def test_mixin(self):
        @six.add_metaclass(serializers.SerializerMetaclass)
        class MyMixin(object):
            some_field = serializers.BooleanField()

        class MySerializer(MyMixin, serializers.Serializer):
            class Meta:
                fields = ['some_field2']

        serializer = MySerializer(data={})
        serializer.is_valid()
        assert 'some_field' in serializer.errors

I more asking if this from a framework design perspective is ok or if the meta class is considered internal.

[xordoquy]

The metaclass is considered as internal since it's not documented. However, it's unlikely to change.

[rpkilby]

Why do

@six.add_metaclass(serializers.SerializerMetaclass)
class MyMixin(object):

when you can just do

class MyMixin(serializers.Serializer):

especially since the former is not technically a public API? Is there something I'm missing? ModelSerializer inherits from Serializer.

[vesterbaek]

I'm using this for both Serializer and ModelSerializer. Not sure about the consequences for multiple inheritance like that. Guess it should work if the mixin class is listed after the ModelSerializer

[xordoquy]

Using Serializer instead of just the meta may mess with heritage if some intermediate mixins do override some members.

[ThatsAMorais]

I know one thing that could be problematic when inheriting serializers.Serializer is having to add definitions for all abstract methods of Serializer in said mixin. I used the metaclass and was successful in achieving what I wanted.