Django中使用分组进行条件批量更新

Question

假设我有一个交易列表，其模型定义如下:

class Transaction(models.Model):
    amount = models.FloatField()
    client = models.ForeignKey(Client)
    date = models.DateField()
    description = models.CharField()
    invoice = models.ForeignKey(Invoice, null=True)

现在我想在月底为每个客户创建发票。发票模型如下所示：

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)

我知道使用批量创建可以在1个查询中创建所有发票，但我仍然需要逐个设置交易模型中的发票字段。

是否可能在创建所有发票后，使用单个查询设置所有Transaction模型的发票字段？最好使用ORM，但如果需要也可以使用原始SQL。

我知道也可以在交易列表上按客户分组以获得每个客户的总额，但是这样的话，各个条目将无法链接到发票。

- Kritz

Answer 1

如果您能在之前生成从客户到发票的映射，那么您可以尝试构建条件更新查询：

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))

请注意，条件查询自Django 1.8版本开始可用。否则，您可以考虑使用原始SQL构建类似的查询。

- Bernhard Vallant

Answer 2

为了补充@Bernhard Vallant的答案，您只需要使用3个查询。

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))

- Alexandr Tatarinov