Question

所以我有一个注释模型并通过查询

comments = Comments.objects.values('students_id', 'created_at')

我得到这个输出

<QuerySet [
{'students_id': 4, 'created_at': datetime.date(2019, 6, 19)}, {'students_id': 2, 'created_at': datetime.date(2019, 6, 3)}, {'students_id': 1, 'created_at': datetime.date(2019, 6, 24)}, {'students_id': 6, 'created_at': datetime.date(2019, 6, 4)}, {'students_id': 6, 'created_at': datetime.date(2019, 6, 19)}, {'students_id': 5, 'created_at': datetime.date(2019, 6, 5)}, {'students_id': 4, 'created_at': datetime.date(2019, 7, 28)}, {'students_id': 6, 'created_at': datetime.date(2019, 6, 11)}]>

id = 6的学生有3条评论，id = 4的学生有2条评论。我需要得到的只是每个学生的一个最新评论。在此示例中，它将如下所示：

<QuerySet [
{'students_id': 2, 'created_at': datetime.date(2019, 6, 3)}, {'students_id': 1, 'created_at': datetime.date(2019, 6, 24)}, {'students_id': 6, 'created_at': datetime.date(2019, 6, 19)}, {'students_id': 5, 'created_at': datetime.date(2019, 6, 5)}, {'students_id': 4, 'created_at': datetime.date(2019, 7, 28)},]>

提前感谢您的回答！

Answer 1

您可以使用注释和最大值来获得所需的结果，例如Comments.objects.values('students_id').annotate(Max('created_at')) 输出将类似于<QuerySet [ {'students_id': 2, 'created_at__max': datetime.date(2019, 6, 3)}, {'students_id': 1, 'created_at__max': datetime.date(2019, 6, 24)},]>，其中包含students_id和最新created_at。要使用此功能，您必须从django.db.models这样的from django.db.models import Max

导入Max

Answer 2

使用此代码：

queryset=Comments.objects.values('students_id', 'created_at').group_by('students_id').annotate(Latest_created_at=Max('created_at'))
queryset.delete()

Answer 3

在原始SQL中，它将为... WHERE NOT EXISTS(SELECT * FROM Comments cc WHERE cc.student_id = c.student_id AND cc.created_at > c.created_at)

later_comments = Comments.objects.filter(student_id=OuterRef('student_id'), 
    created_at__gt=OuterRef('created_at'), ).values('created_at', )

latest_comments = Comments.objects.\
    annotate(has_later_comments=Exists(later_comments), ).\
    filter(has_later_comments=False, )

如果您的created_at是Date列（没有时间），则您需要使用=>而不是>，因为在此期间可能会创建多个注释一天。因此，查询将包含带有附加列的附加谓词，用于对注释进行排序（例如ID）：WHERE cc.created_at > c.created_at OR cc.created_at = c.created_at AND cc.id > c.id

https://docs.djangoproject.com/en/2.2/ref/models/expressions/#exists-subqueries

如果日期字段大于其他重复字段，如何排除具有相同字段的项目？

3 个答案: