(Is it a bug?) TruncMonth does not work when there is "ordering" in the model Meta (Django 1.10)

[Neto]

I'm trying to get number of records per month, but Django does not return correctly when there is "ordering" in class Meta Model.

I have the Abc model, with 4 records made this month, look what happens (Django 1.10, PostgreSQL):

class Abc(Base):
    
    class Meta:
        ordering = ['-date_pub']

>>> Abc.objects.annotate(month=TruncMonth('created_at')).values('month').annotate(total=Count('id')).values('total')
<QuerySet [{'total': 1}, {'total': 1}, {'total': 1}, {'total': 1}]>

Without "ordering"

class Abc(Base):
    pass

>>> Abc.objects.annotate(month=TruncMonth('created_at')).values('month').annotate(total=Count('id')).values('total')
<QuerySet [{'total': 4}]>

Why this happens? Is it a bug?

[Tim Graham]

Default ordering affects aggregation queries like that: https://docs.djangoproject.com/en/stable/topics/db/aggregation/#interaction-with-default-ordering-or-order-by

[Neto]

Thanks! Should have a note about it on the page "database-funcions":

https://docs.djangoproject.com/en/1.10/ref/models/database-functions/#trunc

[Tim Graham]

Unfortunately, I don't think mentioning the issue every place where someone might be doing an aggregation query is practical. I did recently add a link about it to the Meta.ordering docs recently [0], so hopefully that helps raise awareness.

[0] https://github.com/django/django/commit/3aaf6cf0f3288986c4ce56defea26420f8a48534