Ordering by a field in reverse side foreign key in Django

I have the following models

class Question(models.Model):
    content=models.TextField()

    ...



class Answer(models.Model):
    created_at = models.DatetimeField(auto_add=True)
    author = models.ForeignKey(User, ... )
    content = models.TextField()
    question = models.ForeignKey(Question, ....)

    ...

    class meta:
        unique_together=('author','question')

in the database I have a set of questions and answers. answers are linked to their authors. unique_together ensures the user can give an answer only once to a question.

usr_1 Is a user instance that has answers to some questions that I can retrieve by:

qst_qs=Question.objects.filter(answer__author=usr_1)

I want to sort questions in qst_qs according to the created_at field in usr_1's answer in each question.

Thanks in advance.

Use an annotation for usr_1’s answer timestamp, then order by it:

from django.db.models import Max
qst_qs = (Question.objects.filter(answer__author=usr_1)
          .annotate(user_answer_created_at=Max("answer__created_at"))
          .order_by("-user_answer_created_at"))

annotate() adds the per-question answer time (for that user), and order_by() sorts by it.