django: got an unexpected keyword argument while accessing ForeignKey _id-Field in Manager

Question

I have a model which looks like this:

class Mentorship (models.Model):
    mentor = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='mentor_user_id')
    mentee = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='mentee_user_id')

    objects = MentorshipManager()

    def clean(self):
        print(self.mentor_id)  # is set and printed to stdout
        print(self.mentee_id)  # is set and printed to stdout

        if Mentorship.objects.is_mentor(self.mentor_id, self.mentee_id):
            raise ValidationError(_('This user is already a mentor.'))

The manager has a function to check if someone is already a mentor of another user, which is called while clean()ing the instance:

def is_mentor_for_goal(self, mentor_id, mentee_id, personal_goal_id):
    if self.exists(mentor_id=mentor_id, mentee_id=mentee_id):
        return True
    else:
        return False

However I always get an exception while accessing the mentor_id or mentee_id attribute in exists:

Django Version:     1.6.1
Exception Type:     TypeError
Exception Value:    exists() got an unexpected keyword argument 'mentor_id'

Is there a reason why I cannot access the _id field within the manager? I just don't understand why the field is accessible in the (unsaved) instance, but not in the manager.

Answer 1

A few things mentor__id will only work in queryset methods, not things like print. You should also use pk instead of id, here is how it would work:

class Mentorship(models.Model):
    mentor = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='mentor_user_id')
    mentee = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='mentee_user_id')

    objects = MentorshipManager()

    def clean(self):
        print(self.mentor.pk)  # is set and printed to stdout
        print(self.mentee.pk)  # is set and printed to stdout

        if Mentorship.objects.filter(mentor=self.mentor).exists():
            raise ValidationError(_('This user is already a mentor.'))

def is_mentor_for_goal(self, mentor_id, mentee_id, personal_goal_id):
    return self.exists(mentor__pk=mentor_id, mentee__pk=mentee_id)

Answer 2

Oh... man... It should be called

if self.filter(mentor_id=mentor_id, mentee_id=mentee_id).exists()

Thanks for your comments