Use Coalesce to give default values to aggregates

Aggregates return an explicit None if there is no data to aggregate, and dict's get method
only returns teh default if the key does not exist.

Closes #43.

aleksis/apps/chronos/models.py

@@ -7,6 +7,7 @@ from django.core import validators
 from django.core.exceptions import ValidationError
 from django.db import models
 from django.db.models import F, Max, Min, Q
+from django.db.models.functions import Coalesce
 from django.http.request import QueryDict
 from django.utils.decorators import classproperty
 from django.utils.translation import ugettext_lazy as _
@@ -247,27 +248,27 @@ class TimePeriod(models.Model):
 
     @classproperty
     def period_min(cls) -> int:
-        return cls.objects.aggregate(Min("period")).get("period__min", 1)
+        return cls.objects.aggregate(period__min=Coalesce(Min("period"), 1)).get("period__min")
 
     @classproperty
     def period_max(cls) -> int:
-        return cls.objects.aggregate(Max("period")).get("period__max", 7)
+        return cls.objects.aggregate(period__max=Coalesce(Max("period"), 7)).get("period__max")
 
     @classproperty
     def time_min(cls) -> Optional[time]:
-        return cls.objects.aggregate(Min("time_start")).get("time_start__min", None)
+        return cls.objects.aggregate(Min("time_start")).get("time_start__min")
 
     @classproperty
     def time_max(cls) -> Optional[time]:
-        return cls.objects.aggregate(Max("time_start")).get("time_start__max", None)
+        return cls.objects.aggregate(Max("time_start")).get("time_start__max")
 
     @classproperty
     def weekday_min(cls) -> int:
-        return cls.objects.aggregate(Min("weekday")).get("weekday__min", 0)
+        return cls.objects.aggregate(weekday__min=Coalesce(Min("weekday"), 0)).get("weekday__min")
 
     @classproperty
     def weekday_max(cls) -> int:
-        return cls.objects.aggregate(Max("weekday")).get("weekday__max", 6)
+        return cls.objects.aggregate(weekday__max=Coalesce(Max("weekday"), 6)).get("weekday__max")
 
     class Meta:
         unique_together = [["weekday", "period"]]