Conditional bulk update in Django using grouping

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))

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.