django model新建空querySet：models.Atable.objects.none()

queryset有时需要一个空queryset，用于后续用循环并集的初值，举例如下

tables = DataFlowTable.objects.filter(data_status=0)
logs = DataFlowTableTransLog.objects.filter(data_status=0)
schemas = logs.values_list('source_schema', flat=True).distinct()
# 建一个空queryset，作为后续循环做并集的初值
s_tables = tables.none()
for schema in schemas:
    s_logs = logs.filter(source_schema=schema)
    s_tables |= tables.filter(name__in=s_logs.values('source_table'))
return s_tables