Django 中的查询集联合

Question

I'm trying to do a simple double entry accounting with Django. Every transaction is currently one object with debit and credit accounts being foreign keys. Now I'm trying to get all transaction objects out so that there's debit and credit object for every transaction and I can easily order and represent every account's transactions and totals in the template. So every transaction object needs to have debit object and credit object in the final outcome. I think in MySQL you could achieve this with something like:

SELECT date, description, amount, debit_account AS account, debit AS 1 FROM Transaction
UNION
SELECT date, description, amount, credit_account AS account, debit AS 0 FROM Transaction

What is the right way to do this with Python's objects in Django? Union seems to override the changes within the for loop I currently have and I end up with no distinction between debit and credit.

models.py

class Transaction(models.Model):
    date = models.DateField()
    description = models.CharField(max_length=150)
    debit_account = models.ForeignKey('ledger.Account', related_name='debit_account', blank=True, null=True, on_delete=models.CASCADE)
    credit_account = models.ForeignKey('ledger.Account', related_name='credit_account', blank=True, null=True, on_delete=models.CASCADE)
    amount = models.DecimalField(max_digits=10, decimal_places=2, default=0)
    debit_in_ledger = models.BooleanField(default=False)


views.py

def ledger(request):

    transaction_list_debit = Transaction.objects.all()
    transaction_list_credit = Transaction.objects.all()

    for a in transaction_list_debit:
        a.debit_in_ledger = True

    transaction_list = transaction_list_debit.union(transaction_list_credit, all=True).order_by('debit_account', 'public_date')

    return render(request, 'pages/ledger.html', {'transaction_list' : transaction_list})

Answer 1

You can do this as follows:

from django.db.models import BooleanField, F, Value

q1 = Transaction.objects.annotate(
    account = F('debit_account')
    debit = Value(True, output_field=BooleanField())
)

q2 = Transaction.objects.annotate(
    account = F('credit_account')
    debit = Value(False, output_field=BooleanField())
)

qs = q1 | q2

where qs is thus our "final" queryset. This QuerySet will contain Transaction objects, and every Transaction object in the queryset will have two extra attributes: account and debit . Note that other attributes like debit_account , etc. will still exist. Furthermore every real Transaction in the database will thus occur twice : once from q1 , and once from q2 .

Furthermore the .account will contain the primary key of the related Account , so not a reference to the Account . You can then fetch the Account with Account.objects.get(pk=some_transaction.account) .