Duplicate entries from m2m relation

Using Django Mystery Errors
1

Hi all,

I’m trying to represent a field mapping in Django and I’m getting strange duplicates when querying the mappings.

After a lot of searching, I found out that it is caused by an ordering property.
My best guess is that this causes a faulty join somewhere, but I do not fully understand it or what I should have done instead.
Could someone please explain me this behaviour?

class RejiMapping(models.Model):
    class Meta:

        #This causes the duplicates
        ordering = ["from_fields__name"]
        #
    
    from_fields = models.ManyToManyField(RejiMappingField, related_name="mapping_tables_from", through='RejiMappingMappingFromField')
    to_fields = models.ManyToManyField(RejiMappingField, related_name="mapping_tables_to", through='RejiMappingMappingToField')
    description = models.TextField(default="", blank=True)

class RejiMappingField(models.Model):
    class Meta:
        ordering = ['name']
    
    name = models.CharField(max_length=60, null=False, blank=False, unique=True)
    description = models.TextField(default="", blank=True)

class RejiMappingMappingField(models.Model):
    class Meta:
        ordering = ["id"]
        unique_together = ("mapping", "field",)
        abstract=True

    mapping = models.ForeignKey(RejiMapping, 
                                          on_delete=models.CASCADE,
                                          verbose_name=_("Mapping"))
    field = models.ForeignKey(RejiMappingField, 
                                       on_delete=models.CASCADE,
                                       verbose_name=_("Field"))

class RejiMappingMappingFromField(RejiMappingMappingField):
    pass

class RejiMappingMappingToField(RejiMappingMappingField):
    pass

class TestRejiMapping(TestCase):
    def setUp(self):
       f1 = RejiMappingField.objects.create(name="Field1")
       f2 = RejiMappingField.objects.create(name="Field2")
       f3 = RejiMappingField.objects.create(name="Field3")

       mapping = RejiMapping.objects.create()
       mapping.from_fields.add(f1, f2)
       mapping.to_fields.add(f3)

    def test_mapping_model(self):
        self.assertEqual(2, RejiMappingMappingFromField.objects.all().count())
        self.assertEqual(1, RejiMappingMappingToField.objects.all().count())
        self.assertEqual(1, RejiMapping.objects.all().count())
        self.assertEqual(RejiMapping.objects.all().count(), len(RejiMapping.objects.all()))

FAIL: test_mapping_model (reji_app.models.tests.test_reji_mapping_model.TestRejiMappingTable)
----------------------------------------------------------------------
Traceback (most recent call last):
    self.assertEqual(RejiMapping.objects.all().count(), len(RejiMapping.objects.all()))
AssertionError: 1 != 2

----------------------------------------------------------------------
Ran 1 test in 0.013s

FAILED (failures=1)
Destroying test database for alias 'default'...

The weird thing is that the duplicate only occurs when I retrieve a full QuerySet using all(), as soon as I use a count(), filter() or order_by(), the duplicates disappear.