Conditional bulk update in Django using grouping - sql

Question

Suppose I have a list of transactions with the following model definition:
class Transaction(models.Model):
amount = models.FloatField()
client = models.ForeignKey(Client)
date = models.DateField()
description = models.CharField()
invoice = models.ForeignKey(Invoice, null=True)
Now I want to create invoices for each client at the end of the month. The invoice model looks like this:
class Invoice(models.Model):
client = models.ForeignKey(Client)
invoice_date = models.DateField()
invoice_number = models.CharField(unique=True)
def amount_due(self):
return self.transaction_set.aggregate(Sum('amount'))
def create_invoices(invoice_date):
for client in Client.objects.all():
transactions = Transaction.objects.filter(client=client)
if transactions.exists():
invoice = Invoice(client=client, number=get_invoice_number(), date=invoice_date)
invoice.save()
transactions.update(invoice=invoice)
I know I can create all the invoices with a bulk create in 1 query with bulk create, but I would still have to the set the invoice field in the transaction model individually.
Is it possible to set the invoice field of all the Transaction models with a single query after I've created all the invoices? Preferably in using the ORM but happy to use raw SQL if required.
I know I can also use group by client on the transaction list to get the total per client, but then the individual entries are not linked to the invoice.

Answer 1

You could try constructing a conditional update query if you are able to generate a mapping from clients to invoices before:
from django.db.models import Case, Value, When
# generate this after creating the invoices
client_invoice_mapping = {
# client: invoice
}
cases = [When(client_id=client.pk, then=Value(invoice.pk))
for client, invoice in client_invoice_mapping.items()]
Transaction.objects.update(invoice_id=Case(*cases))
Note that Conditional Queries are available since Django 1.8. Otherwise you may look into constructing something similar using raw SQL.

Answer 2

To complement #Bernhard Vallant answer. You can use only 3 queries.
def create_invoices(invoice_date):
# Maybe use Exists clause here instead of subquery,
# do some tests for your case if the query is slow
clients_with_transactions = Client.objects.filter(
id__in=Transaction.objects.values('client')
)
invoices = [
Invoice(client=client, number=get_invoice_number(), date=invoice_date)
for client in clients_with_transactions
]
# With PostgreSQL Django can populate id's here
invoices = Invoice.objects.bulk_create(invoices)
# And now use a conditional update
cases = [
When(client_id=invoice.client_id, then=Value(invoice.pk))
for invoice in invoices
]
Transaction.objects.update(invoice_id=Case(*cases))