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
26430	Coalesce in Aggregations ignored when EmptyResultSet returned	Ryan Prater	Simon Charette	"Using an empty list when using the `__in=` returns an EmptyResultSet and prevents an Aggregate Coalesce from working properly. See below:

# Test with matched Queryset. Sum will return 50
{{{OrderItem.objects.filter(pk__in=[1]).aggregate(test=Coalesce(Sum('quantity'), Value(0)))}}}
{{{>>>{'test': 50}}}}
 
# Test with unmatched Queryset. Sum will return 0
{{{OrderItem.objects.filter(pk__in=[-1]).aggregate(test=Coalesce(Sum('quantity'), Value(0)))}}}
{{{>>> {'test':0}}}}
 
# Test with unmatched Queryset (using empty list). EmptyResultSet returned because of empty list. Sum will return NONE
{{{OrderItem.objects.filter(pk__in=[]).aggregate(test=Coalesce(Sum('quantity'), Value(0)))}}}
{{{>>> {'test': None}}}}

Simon Charette on django-users suggested the following:

''From what I understand the ORM simply doesn't perform any query in this case
as the `pk__in` lookup cannot match any `OrderItem` and result in an
`EmptyResultSet` exception[1].''
''This exception is caught in the `Query.get_aggregation()` method where all
aggregates are converted to `None`[2].''
''I suppose we should alter the `except EmptyResultSet` clause to account for
`outer_query.annotation_select` items that are `Coalesce()` instances used with
`Value()` but I'm unsure about how it should be done.''

[1] https://github.com/django/django/blob/2e0cd26ffb29189add1e0435913fd1490f52b20d/django/db/models/lookups.py#L221-L223
[2] https://github.com/django/django/blob/2e0cd26ffb29189add1e0435913fd1490f52b20d/django/db/models/sql/query.py#L439-L445

See full discussion here:
https://groups.google.com/forum/#!topic/django-users/HGD3Vv3IerA"	Bug	closed	Database layer (models, ORM)	1.9	Normal	fixed	aggregation coalesce in queryset	Simon Charette Hannes Ljungberg Anton Agestam pope1ni	Ready for checkin	1	0	0	0	0	0