How to exclude items with identical field if the datefield is bigger than in others duplicates? - django

Question

So I have a Comments model and by querying
comments = Comments.objects.values('students_id', 'created_at')
I get this output
<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)}]>
It's three comments by student with id=6 and two comments by student with id=4.
What I need to get is only one latest comment from every student. In this example it'll look like this:
<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)},]>
Thanks in advance for the answer!

Answer 1

You can use annotate and max to get desired result like this Comments.objects.values('students_id').annotate(Max('created_at'))
and the output will be like this <QuerySet [
{'students_id': 2, 'created_at__max': datetime.date(2019, 6, 3)}, {'students_id': 1, 'created_at__max': datetime.date(2019, 6, 24)},]> which will have students_id and latest created_at. To use this you have to import Max from django.db.models like this from django.db.models import Max

Answer 2

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

Answer 3

In raw SQL it would be ... 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, )
If your created_at is a Date column (no time), then you need to use => instead of > because perhaps more than one comment can be created during a day. So the query would contain additional predicate with extra column for ordering comments (like 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