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
15183	Annotation count by value always returns zero count for nulls	Sergiy Kuzmenko	nobody	"= How to replicate =

Run count annotation on a field that has nulls in it:

{{{
ann = MyModel.objects.values(""my_field"").annotate(Count(""my_field""))
for a in ann: print a
   ...
{'my_field__count': 0, 'my_field': None}
   ...
}}}

The count of null values always comes as zero regardless of their actual number.


= Cause =

The generated SQL statement uses count clause in the form of {{{ count(<field_name>) }}} which does not count nulls. Unfortunately (at least MySQL and PostgreSQL) backends include null values into output even though nulls are not counted! Which leaves the user with a false impression that there are no null values at all.


= Solution =

The generated SQL statement must be in the form {{{ count(*) }}} instead of {{{ count(<field_name>) }}}. This will result in correct count of nulls.


= Backward compatibility concerns =

This solution may introduce a small backward incompatibility for users (if there are such) relying on the fact that null counts are always zero. If the proposed change is implemented those users will have to apply filtering if counting null values is not desired:

{{{
ann = MyModel.objects.filter(~Q(my_field=None)).values(""my_field"").annotate(Count(""my_field""))
}}}"		closed	Uncategorized	1.2		wontfix		s.kuzmenko@…	Unreviewed	0	0	0	0	0	0