如何在相关的 one_to_many 字段上正确使用 order_by 和 nulls_last？

Question

I'm trying to filter and order objects by existance of a related model with forcing nulls as last.

Example models:

class ModelA(models.Model):
    name = models.CharField(max_length=255)
    ...


class ModelB(models.Model):
    model_a = models.ForeignKey(ModelA, related_name=model_b)
    date = models.DateField()
    time = models.TimeField()
    ...

Now, I need to filter ModelA by some kwargs (normal operations), but I also need to order ModelA by nearest ModelB date and time (in future, not past) ascending or descending WITH A SINGLE QUERY PARAM and also NULLS need to be placed last regardless of ascending or descending order.

I already came to a conclusion that I need to overwrite default QuerySet order_by:

class ModelAQuerySet(models.QuerySet):
    def order_by(self, *field_names):
        if 'model_a_date' in field_names or '-model_a_date' in field_names:
            field_names = list(field_names)
            if 'model_a_date' in field_names:
                field_names.remove('model_a_date')
                return super().order_by(
                    'model_b__date',
                    'model_b_time',
                    *field_names
                )
            else:
                field_names.remove('-model_a_date')
                return super().order_by(
                    '-model_b__date',
                    '-model_b__time',
                    *field_names
                )
        else:
            return super().order_by(*field_names)

It works fine when I'm ordering descending ( '-model_a_date' ), since NULLS are last, but I'm having a hard time placing nulls last in ascending order. I already tried:

return super().order_by(
    F('model_b').asc(nulls_last=True),
    'model_b__date',
    'model_b__time',
    *field_names
)

####

return super().order_by(
    F('model_b').asc(nulls_last=True)
).order_by(
    'model_b__date',
    'model_b_time',
    *field_names
)

####

return super().order_by(
    'model_b__date',
    'model_b_time',
    *field_names
).order_by(
    F('model_b').asc(nulls_last=True),
)

but that does not work at all. Either when I put inside order_by along with other arguments, or as a separate order_by in front or after the above order_by.

Any ideas on that? Has anyone bumped into such problem? I'm in a pickle here and honestly I ran out of ideas. Thanks in advance.

EDIT

I also tried.union like so:

ModelA.objects.annotate(
    modelbs=Count('modelb')
).filter(
    modelbs__gt=0
).union(
    ModelA.objects.annotate(
        modelbs=Count('model_b')
    ).filter(
        modelbs=0
    )
)

But this comes out with an error:

ORDER BY term does not match any column in the result set.

Answer 1

If you use chain you can:

• first filter the elements that are not null order them by ascending or descending order
• second select the nulls
• finally you will be able to chain the to qs that you have (with nulls at the end) and return it

Does that make sense?