Saving twice causes TypeError: Field 'id' expected a number but got ObjectId

[Jibola]

Context

after running python mange.py shell I attempted to query the database I just collected.

>>> from django.utils import timezone
>>> q = Question(question_text="What's new?", pub_date=timezone.now())
>>> q.save()
>>> q.id
ObjectId('66b24df05ad79637c54da774')
>>> q.question_text = "What's up"
>>> q.save()
Traceback (most recent call last):
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/fields/__init__.py", line 2117, in get_prep_value
    return int(value)
           ^^^^^^^^^^
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'ObjectId'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/base.py", line 822, in save
    self.save_base(
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/base.py", line 909, in save_base
    updated = self._save_table(
              ^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/base.py", line 1040, in _save_table
    updated = self._do_update(
              ^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/base.py", line 1084, in _do_update
    filtered = base_qs.filter(pk=pk_val)
               ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/query.py", line 1476, in filter
    return self._filter_or_exclude(False, args, kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/query.py", line 1494, in _filter_or_exclude
    clone._filter_or_exclude_inplace(negate, args, kwargs)
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/query.py", line 1501, in _filter_or_exclude_inplace
    self._query.add_q(Q(*args, **kwargs))
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1613, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1645, in _add_q
    child_clause, needed_inner = self.build_filter(
                                 ^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1559, in build_filter
    condition = self.build_lookup(lookups, col, value)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/sql/query.py", line 1389, in build_lookup
    lookup = lookup_class(lhs, rhs)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/lookups.py", line 30, in __init__
    self.rhs = self.get_prep_lookup()
               ^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/lookups.py", line 364, in get_prep_lookup
    return super().get_prep_lookup()
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/lookups.py", line 88, in get_prep_lookup
    return self.lhs.output_field.get_prep_value(self.rhs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/django/lib/python3.12/site-packages/django/db/models/fields/__init__.py", line 2119, in get_prep_value
    raise e.__class__(
TypeError: Field 'id' expected a number but got ObjectId('66b24df05ad79637c54da774').
>>> q.save()

When I check the database I do see the value, and running this yields:

>>> Question.objects.all().values()
<QuerySet [{'id': None, 'question_text': "What's new?", 'pub_date': datetime.datetime(2024, 8, 6, 16, 23, 10, 65000, tzinfo=datetime.timezone.utc)}]>

[timgraham]

It looks like the model in question is using AutoField rather than MongoAutoField.

Have you followed the README and put DEFAULT_AUTO_FIELD = "django_mongodb.fields.MongoAutoField" in your settings?

[Jibola]

Yeah, that was my first change. I didn't have it on the first go around and then included it and this error still continued.

When I do the .save() the first time, the id field renders as a valid ObjectId. It's only when I try again on the save that I get that type error.

[timgraham]

Then it looks like the setting isn't taking effect. I'm not sure if this is the root cause, but your migrations will be incorrect if you don't add that setting before creating them. You should start again and make sure you see MongoAutoField in the files that makemigrations creates.

[timgraham]

After some offline debugging, the problem is that the line default_auto_field = 'django.db.models.BigAutoField' in polls/apps.py must be removed. PR to document: #96.