Django-rest-framework: Many to Many field with through models not nesting properly.

[neighlyd]

When attempting to serialize a Many to Many field with through models, an error is thrown that the final model object does not have the appropriate attribute.

Steps to reproduce

# models.py
class Person(models.Model):

    name = models.CharField(max_length=50)

    def __str__(self):
        return self.name


class Case(models.Model):

    summary = models.TextField()
    litigant_count = models.ManyToManyField(Person, through='Litigant')

    def __str__(self):
        return 'Case %s' % (self.id)

class Litigant(models.Model):

    person = models.ForeignKey(Person, on_delete=models.CASCADE, related_name='person_to_case')
    case = models.ForeignKey(Case, on_delete=models.CASCADE, related_name='case_to_person')
    role = models.TextField()

# serializers.py
class PersonSerializer(ModelSerializer):

    class Meta:
        model = Person
        fields = ('name')


class LitigantSerializer(ModelSerializer):

    class Meta:
        model = Litigant
        fields = ('person', 'case', 'role')


class CaseSerializer(ModelSerializer):

    queryset = Litigant.objects.all()
    litigant_count = LitigantSerializer(queryset, many=True, read_only=True)

    class Meta:
        model = Case
        fields = ('summary', 'litigant_count')

# views.py

class PersonViewSet(ModelViewSet):
    queryset = Person.objects.all()
    serializer_class = PersonSerializer


class LitigantViewSet(ModelViewSet):
    queryset = Litigant.objects.all()
    serializer_class = LitigantSerializer


class CaseViewSet(ModelViewSet):
    queryset = Case.objects.all()
    serializer_class = CaseSerializer

Expected behavior

I expected a nested serializer like that shown in the documents under Nested Relationships, as modified by the Many to Many with Through Model section indicates.

After searching extensively on the web (Stackoverflow, Reddit, github, and the Google Group), all answers appear to indicate that including many=True should cause the serializers to work (see, for example, this thread). Another thread indicated that the fields in the intermediate model needed to be set explicitly to ReadOnlyField, as though read_only=True was not adequate on its own. Neither of these resolves the issue (though the latter does display the API, it simply truncates the through model entirely from the serialization).

Actual behavior

When accessing the Cases API, it responds with the following error:

'Person' object has no attribute 'person'

The stack trace is as follows:

Traceback:

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/django/core/handlers/exception.py" in inner
  41.             response = get_response(request)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/django/views/decorators/csrf.py" in wrapped_view
  58.         return view_func(*args, **kwargs)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/viewsets.py" in view
  90.             return self.dispatch(request, *args, **kwargs)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/views.py" in dispatch
  489.             response = self.handle_exception(exc)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/views.py" in handle_exception
  449.             self.raise_uncaught_exception(exc)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/views.py" in dispatch
  486.             response = handler(request, *args, **kwargs)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/mixins.py" in list
  48.         return Response(serializer.data)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in data
  739.         ret = super(ListSerializer, self).data

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in data
  263.                 self._data = self.to_representation(self.instance)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in to_representation
  657.             self.child.to_representation(item) for item in iterable

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in <listcomp>
  657.             self.child.to_representation(item) for item in iterable

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in to_representation
  501.                 ret[field.field_name] = field.to_representation(attribute)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in to_representation
  657.             self.child.to_representation(item) for item in iterable

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in <listcomp>
  657.             self.child.to_representation(item) for item in iterable

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/serializers.py" in to_representation
  488.                 attribute = field.get_attribute(instance)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/relations.py" in get_attribute
  179.         return get_attribute(instance, self.source_attrs)

File "/home/vagrant/.virtualenvs/testsite/lib/python3.4/site-packages/rest_framework/fields.py" in get_attribute
  103.                 instance = getattr(instance, attr)

Exception Type: AttributeError at /api/cases/
Exception Value: 'Person' object has no attribute 'person'

Checklist

• [√] I have verified that that issue exists against the master branch of Django REST framework.
• [√] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [√] This is not a usage question. (Those should be directed to the discussion group instead.)
• [√] This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• [√] I have reduced the issue to the simplest possible case.
• [ ] I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

[xordoquy]

Hi,

I usually redirect usage question to the mailing list but this one is trickier and I feel deserve a response here.

On the models, litigant_count points to a Person model. On the other hand your serializer definition for litigant_count points to the LitigantSerializer.

Therefore you get LitigantSerializer trying to serialize a Person instance thus your issue.

[neighlyd]

Thank you for responding. I appreciate it.

So the through model (a.k.a. junction table on the database) needs to be treated as a simple reverse relationship! The ManyToManyField should be ignored entirely as regards the through model, I presume?

This interaction was not obvious to me, since it hadn't dawned on me that Case.litigant_count was a stand-in for a queryset of Person, rather than Litigant (through related_name=case_to_person). Clearly, my own ignorance is showing here.

Is there any way to update the ManyToManyField with a Through Model section of the documentation to reflect this information, so that others can learn from my mistake? I know that I am not the first person to have this problem, and given the feedback I received on other fora (Reddit, Stackoverflow), the solution was not obvious to others either.

I would be happy to invest some time drafting a rough copy for the documentation if that would help.

[xordoquy]

This is the point where is has to go to the discussion forum

[harikvpy]

Perhaps you have figured this out. You can use the source= argument to achieve the same thing. So your example above can be made to work by this:

litigant_count = LitigantSerializer(source='case_to_person', many=True, read_only=True)

And yes, this does merit mention in the documentation.

[Allan-Nava]

How can I create the manyToMany fields like the example ? i have the problem that is not possibile to create with drf

[rpkilby]

Hi @Allan-Nava. As has been pointed out to you several times, the GitHub issue tracker is intended for bug reports and feature requests. If you need help or support, there are a few places to ask questions, such as IRC, Stack Overflow, and the Google Group.