django Model one-to-many save fail - django-models

Question

I am starting developping an app that uses django model ORM. I came upon a strange behavior when creating and saving objects related by a one-to-many relation. I have made two model creation test cases of a simple one-to-many relationship, one is working and the other is failing, but I don't understand why. Here are my models :
class Document(models.Model):
pass
class Section(models.Model):
document = models.ForeignKey('Document',on_delete=models.CASCADE)
Here is the working creation test case (in manage.py shell):
>>> doc = models.Document()
>>> doc.save()
>>> section = models.Section(document=doc)
>>> section.document
<Document: Document object (5)>
>>> section.save()
>>>
And here is the test case that fails:
>>> doc = models.Document()
>>> section = models.Section(document=doc)
>>> section.document
<Document: Document object (None)>
>>> doc.save()
>>> section.document
<Document: Document object (6)>
>>> section.save()
Traceback (most recent call last):
File "<input>", line 1, in <module>
section.save()
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/base.py", line 741, in save
force_update=force_update, update_fields=update_fields)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/base.py", line 779, in save_base
force_update, using, update_fields,
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/base.py", line 870, in _save_table
result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/base.py", line 908, in _do_insert
using=using, raw=raw)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/manager.py", line 82, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/query.py", line 1186, in _insert
return query.get_compiler(using=using).execute_sql(return_id)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/sql/compiler.py", line 1335, in execute_sql
cursor.execute(sql, params)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/backends/utils.py", line 99, in execute
return super().execute(sql, params)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/backends/utils.py", line 67, in execute
return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/backends/utils.py", line 76, in _execute_with_wrappers
return executor(sql, params, many, context)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/backends/utils.py", line 84, in _execute
return self.cursor.execute(sql, params)
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/backends/mysql/base.py", line 76, in execute
raise utils.IntegrityError(*tuple(e.args))
django.db.utils.IntegrityError: (1048, "Column 'document_id' cannot be null")
>>> section.document
Traceback (most recent call last):
File "<input>", line 1, in <module>
section.document
File "/home/philippe/.local/lib/python3.6/site-packages/django/db/models/fields/related_descriptors.py", line 189, in __get__
"%s has no %s." % (self.field.model.__name__, self.field.name)
diagnosisrank.models.common.Section.document.RelatedObjectDoesNotExist: Section has no document.
>>>
The only difference with the working test case is that the related Document model is not saved before instantiating the Section model. However, after saving the Document model, we see that the related document model of the Section model is pointing to the saved Document (its id is set). But when trying to save the Section, the related id is not set and the related model is lost. Why is that? Why do the related Document model instance must be assigned to the Section only after being saved?
To clarify my problem my goal is to collect all info and instantiate all my models in a first step, then saving all to database in a section step. I can still do this in this way: say D as many S, create D and S, save D, assign D in S, save S. But I would prefer to do: create D, create S with related D, save D, save S. Why can't I?
Thanks for any help or insight!
Phil

Answer 1

Well it was a bug and it is fixed: https://github.com/django/django/commit/519016e5f25d7c0a040015724f9920581551cab0
However it is not in the latest stable 2.2.4 that I get out of pip3 ...