Django groupby day 用 0 填充缺失的数据

Question

I want to make a request in Django where I group by day but I want to fill the day where there is no result with 0, is it possible?

# I use the following query
AccessLog
 .objects
 .filter(attempt_time__gte=last_30_days)
 .annotate(day=TruncDay('attempt_time'))
 .values('day', 'username')
 .annotate(c = Count('username'))
 .order_by('day')

Answer 1

No, it is not possible with annotations. Annotations work with the similar types, for example, Coalesce function requires similar types and mixing datetime and numbers will result in a database error. The same for the Case function there is only one output field per result.

The function TruncDay returns a DateTime (in this case) with fields up to Day set to their minimum value, so for instance 2015-06-15 14:30:50.000321+00:00 will be converted to 2015-01-01 00:00:00+00:00 how documentation outlines. And actually annotated value cannot be sometimes integer and sometimes datetime object.

Occasionally to denote that the values are "None" in such situations preferable way would be to set it to the minimal/maximum value (we assume that the value cannot be equal to it), for instance:

AccessLog.objects.filter(
        attempt_time__gte=last_30_days
    ).annotate(
        day=Coalesce(TruncDay('attempt_time'), datetime.min)
    ).values('day', 'username').annotate(
        c=Count('username')
    ).order_by('day')