python - 如何注释 Django QuerySet 中两个字段的最大值

Question

我有一个模型Client，我如何注释然后排序，它的两个字段的最大值:

from django.db import models

class Client(models.Model):
    uploaded_photo_at = models.DateTimeField()
    uploaded_document_at = models.DateTimeField()

以下内容:

Client.objects.annotate(
    latest_activity_at=Max('uploaded_photo_at', 'uploaded_document_at', output_field=DateTimeField())
).order_by('latest_activity_at')

引发此错误:

django.db.utils.ProgrammingError: function max(timestamp with time zone, timestamp with time zone) does not exist
LINE 1: ...oto_at", "clients_client"."uploaded_document_at", MAX("clien...
                                                             ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

如果有帮助的话，我正在使用 Posgresql 和 Django 1.11。

Answer 1

感谢罗伯特的回答，我找到了 Greatest class of Django .

以下作品:

from django.db.models.functions import Greatest

Client.objects.annotate(
    latest_activity_at=Greatest('uploaded_photo_at', 'uploaded_document_at')
).order_by('latest_activity_at')