如何在Django QuerySet中注释两个字段的Max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()

下列:

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.

我正在使用Posgresql和Django 1.11,如果有帮助的话.

Answer 1

感谢罗伯特的回答,我能够找到GreatestDjango的课程.

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')