Model.ManyToManyField.all() gives AttributeError: 'ManyToManyDescriptor' object has no attribute 'all'

Question

I'm using Django 2.0.2, Python 3.6.4 and PyCharm 2017.3.3

Models: (in models.py)

class Position(models.Model):
    title = models.CharField(max_length=50)
    gang = models.ForeignKey(Gang, on_delete=models.CASCADE)
    description = models.TextField(max_length=20000)

    def __str__(self):
        return str(self.title) + ', ' + str(self.gang)

class Application(models.Model):
    positions = models.ManyToManyField(Position)
    applicant = models.ForeignKey(User, on_delete=models.CASCADE)

class Ranking(models.Model):
    position = models.ForeignKey(Position, on_delete=models.CASCADE)
    applicant = models.ForeignKey(User, on_delete=models.CASCADE)
    rank = models.IntegerField(default=3,validators=[
            MaxValueValidator(3),
            MinValueValidator(1)
        ])

Form: (in forms.py)

class RankingForm(forms.ModelForm):
    rank = forms.IntegerField(max_value=3, min_value=1)
    position = forms.ModelMultipleChoiceField(queryset=Application.positions.all())

    class Meta:
        model = Ranking
        exclude = ['applicant']
        fields = ('rank', 'position')

    def __init__(self, *args, **kwargs):
        super(RankingForm, self).__init__(*args, **kwargs)
        self.fields['rank'].widget.attrs.update({'class': 'form-control'})

I keep getting the AttributeError in RankingForm from

"position = forms.ModelMultipleChoiceField(queryset=Application.positions.all())"

When i write

class Application(models.Model):
    ... 

    def __str__(self):
        return str(self.positions.all())

it shows in django-admin as a QuerySet (which works for forms.ModelMultipleChoiceField()), but writing

    class Application(models.Model):
    ... 

    def __str__(self):
        return str(Application.positions.all())

gives me the same error: 'ManyToManyDescriptor' object has no attribute 'all'

Writing

    class RankingForm(forms.ModelForm):
        ...
        position = forms.ModelMultipleChoiceField(queryset=Position.objects.all())

works, but this is not what i want the field to display.

I want to make a ModelMultipleChoiceField() with all the positions from a specific application, but this error keeps getting in the way. It seems that just referencing a model doesn't work, but referencing self does?? Any help is greatly appreciated! :)

Btw, I haven't found any good documentation on this problem, but this seems to be the code for related_descriptors.py where ManyToManyDescriptor is located

Answer 1

Evaluating relationships are done with an instance that is an initialized instance of the class.

An instance of the Application.

application = Application.objects.first()
application.positions.all()

Change the form queryset after initialization.

class RankingForm(forms.ModelForm):
    rank = forms.IntegerField(max_value=3, min_value=1)
    position = forms.ModelMultipleChoiceField(queryset=Positions.objects.none())

    class Meta:
        model = Ranking
        exclude = ['applicant']
        fields = ['rank', 'position']

    def __init__(self, *args, **kwargs):
        super(RankingForm, self).__init__(*args, **kwargs)
        self.fields['rank'].widget.attrs.update({'class': 'form-control'})  
        self.fields['position'].queryset = self.instance.positions.all()