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.

To reproduce the issue, a simplified scenario ():
Suppose you have recipes (e.g. Pasta dough, Tomato Sauce, etc.), which are linked to dishes (e.g. Pasta with tomato sauce) and ingredients (e.g. Flour, Water, Tomatoes). Ingredients are shared across all users, but each user can add a different purchase price to an ingredient (-> cost). We want to sum up the cost of a dish (i.e. summing up the cost of all ingredients across all recipes of a dish).

**models.py**
{{{
class Ingredient(models.Model):
    pass

class IngredientDetail(models.Model):
    ingredient = models.ForeignKey(Ingredient, on_delete=models.CASCADE, related_name='ingredient_details')
    user = models.ForeignKey(User, limit_choices_to={'role': 0}, on_delete=models.CASCADE)
    cost = models.DecimalField(blank=True, null=True, decimal_places=6, max_digits=13)
    cost_unit = models.CharField(choices=UNITS, max_length=15, default='kg')


class Recipe(models.Model):
    dishes = models.ManyToManyField(Dish, related_name='dishes_recipes', through='RecipeDish')
    ingredients = models.ManyToManyField(Ingredient, related_name='ingredients_recipes', through='RecipeIngredient')

class RecipeDish(models.Model):
    recipe = models.ForeignKey(Recipe, on_delete=models.CASCADE, related_name='recipedish_recipe')
    dish = models.ForeignKey(Dish, on_delete=models.CASCADE, related_name='recipedish_dish')


class RecipeIngredient(models.Model):
    recipe = models.ForeignKey(Recipe, on_delete=models.CASCADE, related_name='recipeingredient_recipe')
    ingredient = models.ForeignKey(Ingredient, on_delete=models.CASCADE, related_name='recipeingredient_ingredient')
    quantity = models.DecimalField(blank=True, null=True, decimal_places=2, max_digits=6)
}}}



Different users can attach different IngredientDetail objects to a shared Ingredient objects (one-to-many relationship). The objective is to calculate how many ingredients added to a recipe (many-to-many relationship through RecipeIngredient) don't have a cost value set. To achieve this without using .extra(), we annotate the appropriate cost field to IngredientNutrition, then sum up the missing values using a Sum(), Case() and When() aggregation.

{{{
def aggregate_nutrition_data(recipes, user):
    annotations['cost_value'] = Subquery(
        IngredientDetail.objects.filter(Q(ingredient_id=OuterRef('ingredient_id')) & Q(user=user)).values('cost')[:1]
    )
    ingredients = RecipeIngredient.objects.filter(recipe__in=recipes).annotate(**annotations)

    aggregators['cost_missing'] = Coalesce(Sum(
        Case(
           When(**{'cost_value': None}, then=Value(1)),
           default=Value(0),
           output_field=IntegerField()
       )
    ), Value(0))

    data = ingredients.aggregate(**aggregators)
    return data
}}}

Expected behaviour: a dictionary is returned: {'cost_missing': <value>}, containing as a value the number of missing cost values.Actual behaviour: AssertionError No exception message supplied is returned on the line ''data = ingredients.aggregate(**aggregators)'' (see abbreviated stacktrace below).

When the aggregated field name is set to a field that exists on the model of the queryset (i.e. is not annotated), the aggregation works. For example: instead of using cost_value=None in the When() of the aggregators, using quantity=None works.
Similarly, doing an aggregation over an annotated field without using Case() and When() works. For example:
{{{
    aggregators['cost'] = Coalesce(
        Sum(
            F('cost_value') * F('quantity') / F('cost_quantity')
            output_field=DecimalField()
        ), Value(0)
}}}


----


**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