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
30188	Aggregate annotation, Case() - When(): AssertionError No exception message supplied	Lukas Klement	nobody	"Aggregating annotations works for simple Sum, Count, etc. operations, but fails when the Sum() contains a Case() When() operation.

Example:
Objective: sum the number of None values for the annotated field field_name across a queryset.
{{{
field_name = <name of annotated field>
missing_data_agg=Sum(
    Case(
            When(**{field_name: None}, then=Value(1)),
            default=Value(0),
            output_field=IntegerField()
    )
)
}}}
Expected behaviour: calling queryset.aggregate(missing_data=missing_data_agg) returns a dict {'missing_data': <value>}
Actual behaviour: AssertionError No exception message supplied is returned.

When field_name is set to a field that exists on the model of the queryset, the aggregation works. When field_name is set to a field that is neither in the model of the queryset, nor has it been annotated to the queryset, the exception returned is as expected: Cannot resolve keyword <name of the non existing field> into field . Choices are: <list of model and annotated fields. In this list, the field_name that has been annotated to the queryset is shown -> hence it should work, hence I assume this is a Django bug.

Stacktrace:

{{{
<Line with the .aggregate(...)>
        return query.get_aggregation(self.db, kwargs) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in get_aggregation
                    expression, col_cnt = inner_query.rewrite_cols(expression, col_cnt) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in rewrite_cols
                new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in rewrite_cols
                new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in rewrite_cols
                new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in rewrite_cols
                new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in rewrite_cols
                new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in rewrite_cols
                new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/sql/query.py in rewrite_cols
        annotation.set_source_expressions(new_exprs) ...
/Users/{{path}}/lib/python3.6/site-packages/django/db/models/expressions.py in set_source_expressions
        assert not exprs 
}}}

The database is a PostgreSQL database, the Django version is 2.1.7, the Python version is 3.6.4. The problem also exists under Django version 2.2b1"	Bug	new	Database layer (models, ORM)	2.1	Normal		query, aggregate, case/when		Unreviewed	0	0	0	0	0	0