Get latest payment from related model in Django Admin - django

Question

I'd like to output a table using "Premises" model in Django admin. In addition I'd like include the output in this table of an additional column, say "last utility payment". It is actually a column in a related table. There may be no payment in the database so admin should able to see either an empty cell or the date of payment.
I was able to write a DB query that displays the information I need. Its significant and worked part is given below:
SELECT jp.id,
jp.number apartment,
jp.building_number building,
jp.rent,
jp.arrears,
jpm.last_payment
FROM jasmin_premises jp
LEFT JOIN (
SELECT pm.premises_id,
max(pm.paid) last_payment
FROM jasmin_payment pm
GROUP BY pm.premises_id
) jpm ON jp.id = jpm.premises_id;
And the output is similar to the following:
id | apartment | building | rent | arrears | last_payment
--------------------------------------------------------------
170 | 1X | 6-A | 297.43 | 2.57, | NULL
72 | 2 | 4 | 289.66 | -678.38 | 2021-01-31
173 | 3Z | 7 | 432.86 | 515.72 | 2021-02-04
73 | 4 | 8-B | 292.25 | 515.44 | 2021-02-04
74 | 5 | 8-B | 112.42 | 3249.34 | NULL
75 | 6A | 122 | 328.48 | 386.23 | 2021-02-04
76 | 7 | 42 | 482.06 | 964.12 | 2021-01-31
77 | 8 | 1 | 433.71 | 867.42 | 2021-01-31
78 | 9C | 12 | 322.79 | 322.79 | 2021-02-04
79 | 10 | 122 | 324.22 | 0 | 2021-02-04
80 | 12 | 12 | 322.79 | 1232.46 | NULL
81 | 14 | 5-Z | 440.82 | 978.44 | 2021-02-04
And I'm using the following models (only a significant part):
class Premises(models.Model):
number = models.CharField(
blank=False,
null=False,
max_length=10)
building_number = models.CharField(
blank=False,
null=False,
max_length=3)
rent = models.DecimalField(
blank=False,
null=False,
max_digits=12,
decimal_places=2,
default=0.0)
area = models.DecimalField(
blank=False,
null=False,
max_digits=5,
decimal_places=2)
class Payment(models.Model):
paid = models.DateField(
blank=False,
null=False)
premises = models.ForeignKey(
Premises,
blank=False,
null=False,
on_delete=models.CASCADE,
related_name='payments',
db_index=True)
Is there a way to override admin.ModelAdmin.get_queryset (for example using annotations) to get an extra column like in my example above? Is there any other way to make a LEFT JOIN on a compound DB query using Django ORM?

Answer 1

to make this query in django you have to add the models.Manager() to the tables like this:
models.py
class Premises(models.Model):
# existent code
objects = models.Manager()
class Payment(models.Model):
# existent code
objects = models.Manager()
In the part of app you want to access this information
from .models import Premises, Payment
premises = Premises.objects.all()
data_to_display = []
for premise in premises:
payments = Payment.objects.filter(premises=premise).order_by('-paid')
if len(payments) == 0:
last_payment = "Null"
else:
last_payment = payments[0]
object_to_list = {
"id": premise.id,
"apartment": premise.number,
"building": premise.building_number,
"rent": premise.rent,
"arreaars": premise.area,
"last_payment": last_payment.paid
}
data_to_display.append(object_to_list)

Answer 2

The solution is to add an explicit subquery to a QuerySet using the Subquery expression. We'll need also to use OuterRef because a queryset in a Subquery needs to refer to a field from the outer query.
So let's create a subquery:
from django.db.models import OuterRef
payments = Payment.objects.filter(
premises=OuterRef('pk')
).order_by('-paid')
The next step is to pass payments subquery to a queryset:
from django.db.models import Subquery
# 'payments' here is from example above
premises = Premises.objects.annotate(
last_payment=Subquery(payments.values('paid')[:1])
)
Finally, lets see the used SQL to query objects rows the database:
print(premises.query)
(the output is formatted, only the significant part is shown)
SELECT "jasmin_premises"."id",
"jasmin_premises"."number",
"jasmin_premises"."building_number",
"jasmin_premises"."arrears",
"jasmin_premises"."rent",
(SELECT U0."paid"
FROM "jasmin_payment" U0
WHERE U0."premises_id" = "jasmin_premises"."id"
ORDER BY U0."paid" DESC
LIMIT 1) AS "last_payment"
FROM "jasmin_premises";
Now, after performing the tests, we can use this in our ModelAdmin:
from django.contrib import admin
from django.db.models import OuterRef, Subquery
from .models import Payment, Premises
#admin.register(Premises)
class PremisesAdmin(admin.ModelAdmin):
list_display = (
'number',
'building_number',
'rent',
'arrears',
'last_payment',
)
def get_queryset(self, request):
qs = super().get_queryset(request)
payments = Payment.objects.filter(
premises=OuterRef('pk')
).order_by('-paid')
qs = qs.annotate(
last_payment=Subquery(payments.values('paid')[:1]),
)
return qs
def last_payment(self, obj):
return obj.last_payment
last_payment.short_description = 'Last payment'
last_payment.admin_order_field = 'last_payment'
Well, this doesn't use JOINs, but this approach will force Django to execute a subquery.
Probably in some cases it may be possible to write an equivalent queryset that performs the same task more clearly or efficiently however, this is the best I have achieved so far.