Django / Python：在第一个外键关系实例上按字段对查询集进行排序

Question

I have the following 2 models, a Lesson model, that can have multiple start / end dates - each with their own Date model.

class Lesson(models.Model):
    name = models.Charfield()
    (...)


class Date(models.Model):
    meeting = models.ForeignKey(
        Lesson,
        verbose_name="dates",
        on_delete=models.CASCADE,
    )
    start_date = models.DateTimeField(
        blank=True,
        null=True,
    )
    end_date = models.DateTimeField(
        blank=True,
        null=True,
    )

For our project we need to order the Lessons by the start_date of their earliest Date instance.

So far example, if I have the following data:

MEETING A:
   DATE_1a: 2022-11-01 --> 2022-11-02
   DATE_2a: 2022-12-10 --> 2022-12-11

MEETING B:
   DATE_1b: 2022-11-03 --> 2022-11-04
   DATE_2b: 2022-11-05 --> 2022-11-06

Then the queryset should return [<MEETING A>, <MEETING B>] based on the start_date values of the DATE_1a/b instances. ( 2022-11-01 is earlier than 2022-11-03 , al other Dates are ignored)

However, what I thought would be a relatively simply query has proven to be a rather evasive problem. My initial approach of the default order_by method used all Date instances attached to a Meeting (and worse, returned a separate Meeting result for each Date instance)

qs.order_by(F("dates__start_date").asc(nulls_first=True))

However -I could not conceive of a query to do what we need to happen. My current working solution is to use a whole bunch of python code to instead do:

ordered_meetings = []
meeting_dict = {}
for meeting in qs:
    first_date = meeting.dates.first()
    if not meeting_dict.get(meeting.id):
        meeting_dict[meeting.id] = first_date.start_date

And then ordering the dict based on its date values, using the ID/KEYS to fetch the Meeting instances and append them to the ordered_meetings list.

But this is such a convoluted (and much slower) approach that I feel that I must be missing something. Does anyone know a more succinct method of accomplishing what we want here?

Answer 1

I'm assuming you need a min?

Something like (untested):

Meeting.objects.annotate(first_date=Min('dates__start_date')).order_by('first_date')