id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
34195	Duplicate Records created when specifying None as a target in a custom ManyToManyField	Credentive	nobody	"Reading through the documents, the only reference to null in ManyToManyFields is the following:

**null has no effect since there is no way to require a relationship at the database level.**

When allowing null values as a target in an M2M field, you are allowed to assign ""None"" as the target of a model. However, if you assign ""None"" multiple times, you will get multiple DB records.

Understanding that M2M fields are implemented as join tables, I can see why this may be happening, but I think this behavior should be documented at least. Note from the example that adding non-null targets multiple times produces the expected result (it works, but no extra rows are created)

**Model code (<ProjectRoot>/policypublisher/models.py):**


{{{
class Section(models.Model):
    uuid = models.UUIDField(
        primary_key=True,
        unique=True,
        editable=False,
        default=uuid.uuid4,
        help_text=""A unique identifier for the Section"",
    )
    <...>
    version = models.ManyToManyField(Version, related_name=""sections_in_version"")
    <...>
    under = models.ManyToManyField(""self"", through=""SectionHierarchy"", symmetrical=False, related_name=""over"")

class SectionHierarchy(models.Model):
    under_id = models.ForeignKey(Section, on_delete=models.CASCADE, related_name=""+"")
    over_id = models.ForeignKey(Section, null=True, on_delete=models.CASCADE, related_name=""+"")
    version = models.ForeignKey(Version, on_delete=models.CASCADE)

    class Meta:
        constraints = [
            models.UniqueConstraint(fields=[""under_id"", ""over_id"", ""version""], name=""unique_sec_under_per_version"")
        ]

}}}


**$ python manage.py shell**

{{{
Python 3.9.15 (main, Nov 15 2022, 09:54:34) 
[GCC 10.2.1 20210110] on linux
Type ""help"", ""copyright"", ""credits"" or ""license"" for more information.
(InteractiveConsole)
>>> from policypublisher.models import *
>>> section = Section.objects.first()
>>> version = section.version.first()
>>> SectionHierarchy.objects.count()
0
>>> section.under.add(None, through_defaults={""version"": version})
>>> section.under.add(None, through_defaults={""version"": version})
>>> SectionHierarchy.objects.count()
2
>>> section2 = Section.objects.last()
>>> section.under.add(section2, through_defaults={""version"":version})
>>> SectionHierarchy.objects.count()
3
>>> section.under.add(section2, through_defaults={""version"":version})
>>> SectionHierarchy.objects.count()
3
>>> 

}}}

**$ python manage.py dbshell**

{{{
SQLite version 3.34.1 2021-01-20 14:10:07
Enter "".help"" for usage hints.
sqlite> select * from policypublisher_sectionhierarchy;
55||055744fc79e24f89a9384d79894eee7a|be8225c85fab4a0ba8ea879a3d992abe
56||055744fc79e24f89a9384d79894eee7a|be8225c85fab4a0ba8ea879a3d992abe

}}}
"	Bug	closed	Database layer (models, ORM)	4.1	Normal	invalid	ManyToManyField, null		Unreviewed	0	0	0	0	0	0