'NoneType' object has no attribute 'unique' BUT I'm not using the 'unique' attribute

Question

I'm build models in Django 1.8, and I'm using abstract inheritance (which I'm assuming is contributing to the problem). I have abstract models and then I have models which are based on those abstract models. I also have ForeignKey and ManyToMany relations between some models.

Everything looks fine, but when I try to syncdb or 'makemigrations blog' I get an AttributeError which says 'NoneType' object has no attribute 'unique'.

I don't know why I'm getting it, and I tried experimenting with different model setups, and I read lots of forum posts, but for now I've hit a wall.

I'll post the traceback and my models below:

MODELS:

indie_db

from django.db import models

class URL(models.Model):
    link = models.CharField(max_length=400)
    name = models.CharField(max_length=200)

    def __str__(self):
        return self.name


class Artist(models.Model):
    name = models.CharField(max_length=200)
    description = models.TextField(null=True, blank=True)
    link = models.ForeignKey(URL)

    class Meta:
        abstract = True
        ordering = ['name']

    def __str__(self):
        return self.name


class ArtistSingle(Artist):
    birthdate = models.DateField(null=True, blank=True)
    deathdate = models.DateField(null=True, blank=True)


class ArtistGroup(Artist):
    members = models.ManyToManyField(ArtistSingle)
    established = models.DateField(null=True, blank=True)
    disbanded = models.DateField(null=True, blank=True)


class Contributor(models.Model):
    contributing_artist = models.ForeignKey(ArtistSingle, null=True, blank=True)
    alternate_name = models.CharField(max_length=200)
    role = models.CharField(max_length=200)


class ProductionCompany(models.Model):
    name = models.CharField(max_length=200)
    link = models.ForeignKey(URL)

    def __str__(self):
        return self.name


class Work(models.Model):
    title = models.CharField(max_length=200)
    description = models.TextField(null=True, blank=True)
    created = models.DateField()
    city = models.CharField(max_length=200)
    production_company = models.ForeignKey(ProductionCompany, blank=True, null=True)
    self_published = models.BooleanField(default=False)
    creator = models.ForeignKey(Artist)
    link = models.ForeignKey(URL)
    styles = models.CharField(max_length=200)
    contributors = models.ManyToManyField(Contributor)

    class Meta:
        abstract = True
        ordering = ['-created']

    def __str__(self):
        return self.title


class MusicalWork(Work):
    audio_links = models.ManyToManyField(URL)


class WrittenWork(Work):
    excerpt = models.TextField(null=True, blank=True)


class PerformanceWork(Work):
    venue = models.CharField(max_length=200)


class VideoWork(Work):
    length = models.CharField(max_length=16)


class VisualWork(Work):
    images = models.ManyToManyField(URL)

blog:

from django.db import models

# Create your models here.


class Tag(models.Model):
    tag_name = models.CharField(max_length=200)

    def __str__(self):
        return self.tag_name


class Entry(models.Model):
    title = models.CharField(max_length=200)
    body = models.TextField()
    slug = models.SlugField(max_length=200)
    publish = models.BooleanField(default=True)
    created = models.DateTimeField(auto_now=True)
    modified = models.DateTimeField(auto_now=True)
    tags = models.ManyToManyField(Tag)

    def __str__(self):
        return self.title

    class Meta:
        verbose_name = "Blog Entry"
        verbose_name_plural = "Blog Entries"
        ordering = ["-created"]

TRACEBACK:

[pattmayne@web476 weird_canada]$ python3.4 manage.py makemigrations blog
Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/core/management/__init__.py", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/core/management/base.py", line 390, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/core/management/base.py", line 440, in execute
    self.check()
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/core/management/base.py", line 478, in check
    include_deployment_checks=include_deployment_checks,
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/core/checks/registry.py", line 72, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/core/checks/model_checks.py", line 28, in check_all_models
    errors.extend(model.check(**kwargs))
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/db/models/base.py", line 1181, in check
    errors.extend(cls._check_fields(**kwargs))
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/db/models/base.py", line 1258, in _check_fields
    errors.extend(field.check(**kwargs))
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/db/models/fields/related.py", line 1829, in check
    errors = super(ForeignKey, self).check(**kwargs)
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/db/models/fields/related.py", line 1502, in check
    errors.extend(self._check_unique_target())
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/db/models/fields/related.py", line 1522, in _check_unique_target
    for rel_field in self.foreign_related_fields)
  File "/home/pattmayne/webapps/limbs_008/lib/python3.4/Django-1.8.2-py3.4.egg/django/db/models/fields/related.py", line 1522, in <genexpr>
    for rel_field in self.foreign_related_fields)
AttributeError: 'NoneType' object has no attribute 'unique'

IF this is caused by my inherited models, what is the exact cause, and how should I change things?

Thanks in advance.

Answer 1

I believe the issue is actually with the Work models.

You have a ForeignKey to URL in the abstract class,

link = models.ForeignKey(URL)

And you also have keys to URL in some of the derived classes, for example MusicalWork:

class MusicalWork(Work):
    audio_links = models.ManyToManyField(URL)

So MusicalWork has two links to URL. Which would be fine, except Django attempts to create a reverse relationship for URL to your model, usually called musicalwork_set for this case, but it has two reverse relationships for the same model!

The answer would be to specify a related_name field for any models that derive from it with the same model references.

Therefore:

class MusicalWork(Work):
    audio_links = models.ManyToManyField(URL, related_name='musicalwork_audio_set')

But there may be other issues as that error message doesn't exactly point to this condition (trust me, Django has a much nicer error message for this particular mistake).