使用分组在Django中进行条件批量更新

Question

假设我有一个具有以下模型定义的事务列表:

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)

我知道我可以通过批量创建在1个查询中创建批量创建的所有发票,但我仍然需要单独在交易模型中设置发票字段.

在创建所有发票后,是否可以使用单个查询设置所有交易模型的发票字段？最好在使用ORM时,但如果需要,很乐意使用原始SQL.

我知道我也可以在交易清单上使用逐个客户来获取每个客户的总数,但随后各个条目都没有链接到发票.

Answer 1

如果您之前能够生成从客户到发票的映射，则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，条件查询从 Django 1.8 开始可用。否则，您可能会考虑使用原始 SQL 构建类似的东西。

Answer 2

补充@Bernhard Vallant的答案.您只能使用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))