如何在Django中执行双重步骤子查询？

Question

I have the below models:

class Group(models.Model):
    group_name = models.CharField(max_length=32)

class Ledger(models.Model):
    ledger_name = models.CharField(max_length=32)
    group_name  = models.ForeignKey(Group,on_delete=models.CASCADE,null=True,related_name='ledgergroups')

class Journal(models.Model):
    By              = models.ForeignKey(Ledger,on_delete=models.CASCADE,related_name='Debitledgers')
    To              = models.ForeignKey(Ledger,on_delete=models.CASCADE,related_name='Creditledgers')
    Debit           = models.DecimalField(max_digits=20,decimal_places=2,default=0)
    Credit          = models.DecimalField(max_digits=20,decimal_places=2,default=0)

As you can see the Journal model is related with the Ledger models with a Foreignkey relation which is further related with Group model.

My scenario is kind of complex.

I want to filter the Group objects and their balances ( Balances are the difference between their total Debit and their total Credit ).

I want to filter the total Group names and the subtraction of total Debit and total Credit ..( Debit and Credit are the fields of Journal model ).

Can anyone help me to figure out the above.

I have tried Subqueries before in Django but haven't done a two step Subquery in Django .

Any solution will be helpful.

Thank you

Answer 1

You can use annotations to calculate sums, averages, counts, etc. These annotations can then be used to filter on

from django.db.models import Sum, F    

groups_with_negative_balance = Group.objects.annotate(
    total_debt=Sum('ledgergroups__Debitledgers__Debit'),
    total_credit=Sum('ledgergroups__Creditledgers__Credit'),
).annotate(
    balance=F('total_credit') - F('total_debt')
).filter(
    balance__lt=0
)

Answer 2

try this

from django.db.models import Subquery
>>> users = User.objects.all()
>>> UserParent.objects.filter(user_id__in=Subquery(users.values('id')))
<QuerySet [<UserParent: UserParent object (2)>, <UserParent: UserParent object (5)>, <UserParent: UserParent object (8)>]>