Adjust hiding of overlapping public holidays and absences

Determine by location of employment at the date of absence whether
a public holiday overlaps it.

timed/tracking/models.py

@@ -151,12 +151,23 @@ class Meta:
 
 class AbsenceManager(models.Manager):
     def get_queryset(self):
-        from timed.employment.models import PublicHoliday
+        """Return only absences which do not overlap with public holidays."""
+        from timed.employment.models import Employment, PublicHoliday
+
+        # location of employment at the date of absence
+        location_qs = Employment.objects.filter(
+            (
+                models.Q(end_date__gte=models.OuterRef('date')) |
+                models.Q(end_date__isnull=True)
+            ),
+            start_date__lte=models.OuterRef('date'),
+            user_id=models.OuterRef(models.OuterRef('user'))
+        ).values('location')[:1]
 
         queryset = super().get_queryset()
         queryset = queryset.exclude(date__in=models.Subquery(
             PublicHoliday.objects.filter(
-                location__employments__user=models.OuterRef('user')
+                location=models.Subquery(location_qs)
             ).values('date')
         ))
         return queryset

timed/tracking/tests/test_absence.py

@@ -9,14 +9,25 @@
 
 
 def test_absence_list_authenticated(auth_client):
-    absence = AbsenceFactory.create(user=auth_client.user)
+    # public holidays of previous location may not be hidden
+    absence = AbsenceFactory.create(
+        user=auth_client.user, date=datetime.date(2018, 1, 2)
+    )
+    previous_employment = EmploymentFactory.create(
+        user=absence.user,
+        start_date=datetime.date(2017, 1, 1),
+        end_date=datetime.date(2017, 12, 31)
+    )
+    PublicHolidayFactory.create(
+        date=absence.date, location=previous_employment.location
+    )
 
     # overlapping absence with public holidays need to be hidden
     overlap_absence = AbsenceFactory.create(
         user=auth_client.user, date=datetime.date(2018, 1, 1))
     employment = EmploymentFactory.create(
         user=overlap_absence.user,
-        start_date=datetime.date(2017, 12, 31)
+        start_date=datetime.date(2018, 1, 1)
     )
     PublicHolidayFactory.create(
         date=overlap_absence.date, location=employment.location