Error when using "through" intermediate model

[eudaldt]

I think this was asked before but is it possible to define my own intermediate model with sortedm2m using through? In my case I have this model:

class Sample(models.Model):
   id_sample = models.AutoField(primary_key=True)
   name = models.CharField(unique=True, max_length=20)
   indexes = SortedManyToManyField(Index, through='SamplePoolIndexCand', blank=True)
   pools = SortedManyToManyField(Index, through='SamplePoolIndexCand', blank=True)
   gene_lists = SortedManyToManyField(Index, through='SamplePoolIndexCand', blank=True)

The intermediate table:

class SamplePoolIndexCand(models.Model):
    sample_id = models.ForeignKey(Sample, null=True, blank=True, on_delete=models.CASCADE, db_column='id_sample', 
    verbose_name='Mostra')
    pool_id = models.ForeignKey(Pool, null=True, blank=True, on_delete=models.CASCADE, db_column='id_pool', 
    verbose_name='Pool')    
    index_id = models.ForeignKey(Index, null=True, blank=True, on_delete=models.CASCADE, 
    db_column='id_index', verbose_name='Índex')
    gene_cand_list_id = models.ForeignKey(GeneCandList, null=True, blank=True, on_delete=models.CASCADE, 
    db_column='id_gene_cand_list', verbose_name='Llista de gens candidats')

I got this error message:

AssertionError: The model is used as an intermediate model by '<class 'myproject.models.SamplePoolIndexCand'>' but has no defined '_sort_field_name' attribute

How can I solve this?

[swizzlevixen]

Seems this was discussed in this older issue: #77

Unfortunately I'm not proficient enough in Django to give any advice. I have hit the same problem, trying to upgrade an old project that was working fine with a "through" model with django <= 2.2.x and django-sortedm2m <=1.5.x. I'm upgrading things piece by piece through to the latest versions of Django, Python, etc., but once I upgrade to Django 3.0, I have to upgrade django-sortedm2m to ==3.0.0 for Django 3 support… and then my through model is now broken:

❯ python -Wall manage.py runserver --settings=cinedex.settings.local
Performing system checks...

Exception in thread django-main-thread:
Traceback (most recent call last):
  File "/usr/local/Cellar/[email protected]/3.9.16/Frameworks/Python.framework/Versions/3.9/lib/python3.9/threading.py", line 980, in _bootstrap_inner
    self.run()
  File "/usr/local/Cellar/[email protected]/3.9.16/Frameworks/Python.framework/Versions/3.9/lib/python3.9/threading.py", line 917, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/utils/autoreload.py", line 53, in wrapper
    fn(*args, **kwargs)
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/core/management/commands/runserver.py", line 117, in inner_run
    self.check(display_num_errors=True)
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/core/management/base.py", line 392, in check
    all_issues = self._run_checks(
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/core/management/base.py", line 382, in _run_checks
    return checks.run_checks(**kwargs)
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/core/checks/registry.py", line 72, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/core/checks/model_checks.py", line 34, in check_all_models
    errors.extend(model.check(**kwargs))
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/db/models/base.py", line 1257, in check
    *cls._check_fields(**kwargs),
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/django/db/models/base.py", line 1338, in _check_fields
    errors.extend(field.check(from_model=cls, **kwargs))
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/sortedm2m/fields.py", line 204, in check
    self._check_through_sortedm2m()
  File "/Users/mboszko/.virtualenvs/cinedex-wZ1JZ5_O/lib/python3.9/site-packages/sortedm2m/fields.py", line 213, in _check_through_sortedm2m
    assert hasattr(rel.through, '_sort_field_name'), (
AssertionError: The model is used as an intermediate model by '<class 'theoptical.models.EpisodeGuest'>' but has no defined '_sort_field_name' attribute

If someone might be able to explain how I could fix this, I would every grateful. Otherwise I fear I may have to rebuild my django app from scratch, because there are so may moving parts to upgrade.

[merwok]

Have you tried setting an attribute _sort_field_name on the intermediate model class?

[swizzlevixen]

@merwok Forgive me, I haven't touched this code (or really, Django at all) in about 7–8 years, and it's currently a foreign country to me.

Yes, it looks like if I add the _sort_field_name attribute inside the EpisodeGuest class, that silences the error, but I'm not sure it's operating correctly. Would I then need to add a new field to that intermediate class matching the sort_value name? It doesn't seem to say in the Readme here what kind of field sort_value should be. I'm going on the assumption based on this line here:

django-sortedm2m/sortedm2m/fields.py

Line 291 in 79c434c

sort_field = models.IntegerField(default=0)

Should it be like this in the intermediate model class?

class EpisodeGuest(models.Model):
    _sort_field_name = "sort_value"
    sort_value = models.IntegerField(default=0)

UPDATE: I tried this in my local development environment, made migrations, migrated the database, and even after adding a new EpisodeGuest to an episode in the admin, an inspection of the local database shows all sort_value values for the episode guests are still 0, so I'm not sure that it's actually using this value at all. 🤔

To provide more context in my project: theoptical.models consists of several classes linked to SortedManyToManyFields, with the EpisodeGuest class being one of them. I'll try to present a condensed version of the module with the relevant bits:

# coding=utf-8

from cinefex.models import Entity
from django.db import models
from sortedm2m.fields import SortedManyToManyField
[...]

[...]

class Episode(models.Model):
    # An episode of a podcast.

    [...]

    guests = SortedManyToManyField(
        Entity, through="EpisodeGuest", related_name="episode_guests", blank=True
    )  # Person

    def __repr__(self):
        return self.designation

    def __str__(self):
        return self.designation

class EpisodeGuest(models.Model):
    # Fixing sort field error for `django-sortedm2m` >= 1.5.0
    _sort_field_name = "sort_value"
    # extend these through…
    created = models.DateTimeField(auto_now_add=True, auto_now=False)
    updated = models.DateTimeField(auto_now_add=False, auto_now=True)
    episode = models.ForeignKey(Episode, on_delete=models.CASCADE)
    guest = models.ForeignKey(Entity, on_delete=models.CASCADE)
    guest_blurb = models.TextField(help_text="Markdown blurb.", blank=True)
    guest_links = models.ManyToManyField(ExternalLink, blank=True)
    # Special Guests get big headshots in the center column.
    is_special = models.BooleanField(
        default=False, help_text="Usually only one per episode."
    )

    def __repr__(self):
        return f"{self.episode.designation}: {self.guest.name}"

    def __str__(self):
        return f"{self.episode.designation}: {self.guest.name}"

Thanks for taking a look!