django ORM 在外键上加入两个 model

Question

The title might be misleading, I'm not sure on how to express my problem

I have two models declared this way:

class Record(models.Model):
    # autogen id as primary key
    name = models.ForeignKey(Product, on_delete=models.DO_NOTHING)
    value = models.FloatField()
    date = models.DateTimeField()


class Integration(models.Model):
    # autogen id as primary key
    action = models.FloatField()
    on_record = models.ForeignKey(Record, on_delete=models.DO_NOTHING)

I have one Record per hour per day on an entire year, thus 24*365.
For each Record , there are exactely ten (10) Integration s, each having their distinct action .

What I need is a QuerySet on Record that has an "extra column" with the average of the related 10 Integration's action s. So

Record

id	name	value	date
1	A	120.00	2020-01-01 00:00
2	B	85.00	2020-01-01 01:00

Integration

id	action	on_record
0x1	7.00	1
0x2	2.00	1
0x3	9.00	1
...	...	...
0x10	8.41	1
0x11	8.99	2 <-- next Record

What I need

id	name	value	date	integration_avg_action
1	A	120.00	2020-01-01 00:00	8.17
2	B	85.00	2020-01-01 00:00	7.58

I was able to generate the average of the Integration s this way

Integration.objects
    .values('on_record')
    .annotate(avg=Avg('action'))

but this is as far as I've come to. I've tried itertools.chain and many select_related() but I'm more confused now than before. It looks to me as a simple JOIN but I can't find a way to get it via ORM.

Answer 1

Since you want a QuerySet of Record you should annotate on that instead of on Integration . See the section Following relationships backwards in the documentation for aggregation.

Record.objects.annotate(integration_avg_action=Avg('integration__action'))

Answer 2

If at all changing your models is not costly then you can add related name to your FK field:

class Record(models.Model):
    # autogen id as primary key
    name = models.ForeignKey(Product, on_delete=models.DO_NOTHING)
    value = models.FloatField()
    date = models.DateTimeField()


class Integration(models.Model):
    # autogen id as primary key
    action = models.FloatField()
    on_record = models.ForeignKey(Record, on_delete=models.DO_NOTHING, related_name='integration')

Then you can execute the following query:

Record.objects.all().annotate(integration_avg_action=Avg('integration__action')).values('name', 'value', 'date', 'integration_avg_action')