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
34564	returning None instead of zero in Count annotation	Amin Aminian	Amin Aminian	"I am trying to upgrade the Django's version from 3.2 to 4.2 in my project. after installation, some of my tests were broken.
The problem:
I am annotating a queryset with Count function, and the count result in some objects should be zero.
This is my code:
{{{
Order.objects.annotate(count=Count(""customer"", distinct=True))
}}}

In Django==3.2, if some orders don't have any customers, the count field would be zero (as I want).
But in Django==4.2, if some orders don't have any customers, the count field would be None, which breaks my code.
I read the source code for both versions. In version 3.2, we have a method called `convert_value` in `Count` class in `aggregates` module. The method is:

{{{
class Count(Aggregate):
    ...
    def convert_value(self, value, expression, connection):
            return 0 if value is None else value
}}}
As we can see, this function is responsible to return 0 instead of None.
But this method is deleted in version 4.2, and the conversion is handling in `convert_value` property in `BaseExpression` class in `expressions` module:

{{{
class BaseExpression:
    ...
    @cached_property
    def convert_value(self):
        field = self.output_field
        internal_type = field.get_internal_type()
        if internal_type == ""FloatField"":
            return (
                lambda value, expression, connection: None
                if value is None
                else float(value)
            )
        elif internal_type.endswith(""IntegerField""):
            return (
                lambda value, expression, connection: None
                if value is None
                else int(value)
            )
        elif internal_type == ""DecimalField"":
            return (
                lambda value, expression, connection: None
                if value is None
                else Decimal(value)
            )
        return self._convert_value_noop
}}}
In this property we are return a lambda function to be called later, and when we call this lambda, what ever we pass to it, we would get None as the result.
Should it be something like returning 0, if the value is None ?"	Bug	closed	Database layer (models, ORM)	4.2	Normal	invalid	count, orm, annotate	Simon Charette	Accepted	1	0	1	1	0	0