retrieve_related fails for toOne relationship

[n2ygk]

@Anton-Shutik Using your new #451 support for related links, using{'get': 'retrieve_related'} works fine for a serializer ResourceRelatedField where many=True but fails for a toOne relationship where many=False.

Using the old style, I had to override get_queryset() for many=True views and get_object() for many=False views so I expect there is a similar need for another "flavor" of retrieve_related for toOne relationships. Does this make sense?

Here's my old view code for reference:

class CourseTermViewSet(CourseBaseViewSet):
    queryset = CourseTerm.objects.all()
    serializer_class = CourseTermSerializer

    def get_queryset(self, *args, **kwargs):
        """
        Implement `related` view:
        Override `.list` if course_pk kwarg is present since course is toMany to course_term.
        """
        course_pk = self.kwargs.get('course_pk', None)
        if course_pk is not None:
            return self.queryset.filter(course_id=course_pk)
        return super(CourseTermViewSet, self).get_queryset()

class CourseViewSet(CourseBaseViewSet):
    queryset = Course.objects.all()
    serializer_class = CourseSerializer

    def get_object(self):
        """
        Implement `related` view:
        Override `.retrieve` if course_term_pk kwarg is present since course_term is toOne to course.
        """
        course_term_pk = self.kwargs.get('course_term_pk', None)
        if course_term_pk is not None:
            return CourseTerm.objects.get(id=course_term_pk).course
        return super(CourseViewSet, self).get_object()

The new code (eliminates those overrides, sets the kwarg to pk and updates the related_link_view_names to course-related and course_term-related, respectively and throws this error when retrieved:

AttributeError at /v1/course_terms/a1d34785-cc25-4c1c-9806-9d05a98068c7/course/
Got AttributeError when attempting to get a value for field `school_bulletin_prefix_code` on serializer `CourseSerializer`.
The serializer field might be named incorrectly and not match any attribute or key on the `PKOnlyObject` instance.
Original exception text was: 'PKOnlyObject' object has no attribute 'school_bulletin_prefix_code'.

...
Traceback:

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/fields.py" in get_attribute
  441.             return get_attribute(instance, self.source_attrs)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/fields.py" in get_attribute
  100.                 instance = getattr(instance, attr)

During handling of the above exception ('PKOnlyObject' object has no attribute 'school_bulletin_prefix_code'), another exception occurred:

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
  34.             response = get_response(request)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  126.                 response = self.process_exception_by_middleware(e, request)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  124.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/django/views/decorators/csrf.py" in wrapped_view
  54.         return view_func(*args, **kwargs)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/viewsets.py" in view
  103.             return self.dispatch(request, *args, **kwargs)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/views.py" in dispatch
  483.             response = self.handle_exception(exc)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/views.py" in handle_exception
  443.             self.raise_uncaught_exception(exc)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/views.py" in dispatch
  480.             response = handler(request, *args, **kwargs)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework_json_api/views.py" in retrieve_related
  128.         return Response(serializer.data)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/serializers.py" in data
  560.         ret = super(Serializer, self).data

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/serializers.py" in data
  262.                 self._data = self.to_representation(self.instance)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/serializers.py" in to_representation
  514.                 attribute = field.get_attribute(instance)

File "/Users/alan/src/django-training/env/lib/python3.6/site-packages/rest_framework/fields.py" in get_attribute
  462.             raise type(exc)(msg)

[n2ygk]

my old urls looked like this:

    url(r'^v1/courses/(?P<course_pk>[^/.]+)/course_terms/',
        views.CourseTermViewSet.as_view({'get': 'list'}),
        name='course-course_terms'),
    url(r'^v1/course_terms/(?P<course_term_pk>[^/.]+)/course/',
        views.CourseViewSet.as_view({'get': 'retrieve'}), # a toOne relationship
        name='course_terms-course'),

and here are the models:

class Course(CommonModel):
    school_bulletin_prefix_code = models.CharField(max_length=10)
    suffix_two = models.CharField(max_length=2)
    subject_area_code = models.CharField(max_length=10)
    course_number = models.CharField(max_length=10)
    course_identifier = models.CharField(max_length=10, unique=True)
    course_name = models.CharField(max_length=80)
    course_description = models.TextField()

    class Meta:
        ordering = ["course_number"]

    def __str__(self):
        return '%s,%s,%s,%s' % (
            self.id,
            self.course_number,
            self.course_identifier,
            self.course_name
        )


class CourseTerm(CommonModel):
    term_identifier = models.TextField(max_length=10)
    audit_permitted_code = models.PositiveIntegerField(blank=True, default=0)
    exam_credit_flag = models.BooleanField(default=True)
    course = models.ForeignKey('myapp.Course', related_name='course_terms', on_delete=models.CASCADE, null=True,
                               default=None)

    class Meta:
        ordering = ["term_identifier"]

    def __str__(self):
        return '%s,%s,%s' % (self.id, self.term_identifier, self.course.course_identifier)

[n2ygk]

Here's where it happens but not sure what the fix is:

class RelatedMixin(object):
    """
    This mixin handles all related entities, whose Serializers are declared in "related_serializers"
    """

    def retrieve_related(self, request, *args, **kwargs):
        serializer_kwargs = {}
        instance = self.get_related_instance()

        returns PKOnlyObject instead of the full object.
--------
This is because ResourceRelatedField use_pk_only_optimization returns True.
---------

in rest_framework_json_api/views.py L160:

    def get_related_instance(self):
        parent_obj = self.get_object()
        parent_serializer = self.serializer_class(parent_obj)
        field_name = self.get_related_field_name()
        field = parent_serializer.fields.get(field_name, None)

        if field is not None:
            return field.get_attribute(parent_obj)
        else:
            try:
                return getattr(parent_obj, field_name)
            except AttributeError:
                raise NotFound

-------

in rest_framework/relations.py L162: self.use_pk_only_optimization() returns true from ResourceRelatedField.use_pk_only_optimization.

    def get_attribute(self, instance):
        if self.use_pk_only_optimization() and self.source_attrs:
            # Optimized case, return a mock object only containing the pk attribute.
            try:
                instance = get_attribute(instance, self.source_attrs[:-1])
                value = instance.serializable_value(self.source_attrs[-1])
                if is_simple_callable(value):
                    # Handle edge case where the relationship `source` argument
                    # points to a `get_relationship()` method on the model
                    value = value().pk
                return PKOnlyObject(pk=value)
            except AttributeError:
                pass


--------

in rest_framework_json_api/relations.py L197 ResourceRelatedField:

    def use_pk_only_optimization(self):
        # We need the real object to determine its type...
        return self.get_resource_type_from_included_serializer() is not None

[n2ygk]

This works around the problem:

class RRF(ResourceRelatedField):
    def use_pk_only_optimization(self):
        return False

[n2ygk]

Looks like this commit: b4dffb8 in #374

@santiavenda2 perhaps you can take a look.

commit b4dffb8291d5c5ad7e9dae52a6942709082ff77a
Author: santiavenda <santiavenda2@gmail.com>
Date:   Mon Jan 8 18:02:47 2018 -0800

    Avoid query explosion for related fields where I can user pk_only_opt… (#374)
    
    * Fix factory boy dependency
    
    * fix factory-boy version in setup.py and requirements-development.txt
    
    * Fix setup.py factory boy dependency
    
    * Avoid query explosion for related fields where I can user pk_only_optimization
    
    * Fix autoPrefetchMixin for ReverseOneToOneDescriptor
    
    * Fix code style
    
    * Avoid query objects in ModelSerializer to_representation method
    
    * Fix code queality error
    
    * Fix problem that makes None related objects not being renderer
    
    * Fix problem that makes None related objects not being renderer
    
    * _readable_field property is missing in drf<3.2

[Anton-Shutik]

@n2ygk Hi, can you show how the fields are declared on your CourseSerializer ?

[n2ygk]

@Anton-Shutik It's here:

django-rest-framework-json-api/example/serializers.py

Lines 333 to 362 in f02c7c4

	class CourseSerializer(HyperlinkedModelSerializer):
	"""
	(de-)serialize the Course.
	"""
	terms = relations.ResourceRelatedField(
	model=Term,
	many=True,
	read_only=False,
	allow_null=True,
	required=False,
	queryset=Term.objects.all(),
	self_link_view_name='course-relationships',
	related_link_view_name='course-related',
	)
	# 'included' support (also used for `related_serializers` for DJA 2.6.0)
	included_serializers = {
	'terms': 'example.serializers.TermSerializer',
	}
	class Meta:
	model = Course
	fields = (
	'url',
	'school_bulletin_prefix_code', 'suffix_two', 'subject_area_code',
	'course_number', 'course_identifier', 'course_name', 'course_description',
	'effective_start_date', 'effective_end_date',
	'last_mod_user_name', 'last_mod_date',
	'terms')

[n2ygk]

@Anton-Shutik and see also here for the test case:

django-rest-framework-json-api/example/tests/test_views.py

Lines 330 to 355 in f02c7c4

	# the following test reproduces/confirm fix for this bug:
	# https://github.com/django-json-api/django-rest-framework-json-api/issues/489
	def test_term_related_course(self):
	"""
	confirm that the related child data reference the parent
	"""
	term_id = self.term.first().pk
	kwargs = {'pk': term_id, 'related_field': 'course'}
	url = reverse('term-related', kwargs=kwargs)
	with mock.patch('rest_framework_json_api.views.RelatedMixin.override_pk_only_optimization',
	True):
	resp = self.client.get(url)
	self.assertEqual(resp.status_code, 200)
	dja_response = resp.json()
	back_reference = dja_response['data']['relationships']['terms']['data']
	self.assertIn({"type": "terms", "id": str(term_id)}, back_reference)
	# the following raises AttributeError:
	with self.assertRaises(AttributeError) as ae:
	with mock.patch(
	'rest_framework_json_api.views.RelatedMixin.override_pk_only_optimization',
	False):
	resp = self.client.get(url)
	print(ae.exception)
	self.assertIn('`PKOnlyObject`', ae.exception.args[0])