django - Django迁移时如何设置/提供默认值？

Question

场景：
我有一个模型，Customer

class Customer(models.Model):
    name = models.CharField(max_length=100)
    age = models.IntegerField()
    company = models.CharField(max_length=100)

现在，我通过如下所示的 company关系更新了 ForeignKey属性，

class Company(models.Model):
    name = models.CharField(max_length=100)
    location = models.CharField(max_length=100)


class Customer(models.Model):
    name = models.CharField(max_length=100)
    age = models.IntegerField()
    company = models.ForeignKey(Company)

我需要的是，当新迁移应用于数据库时，相应的 Company实例必须自动生成并映射到 company实例的 Customer属性。这可能吗？我怎样才能做到这一点？

Answer 1

让我们从原始模型开始，逐步进行。

class Customer(models.Model):
    name = models.CharField(max_length=100)
    age = models.IntegerField()
    company = models.CharField(max_length=100)

首先，您必须保留原始字段并创建一个新字段，然后才能还原旧数据。

class Customer(models.Model):
    name = models.CharField(max_length=100)
    age = models.IntegerField()
    company = models.CharField(max_length=100)
    _company = models.ForeignKey(Company)

现在，您可以使用 manage.py makemigrations创建第一个迁移。然后，您将必须创建一个 data migration。使用 manage.py makemigrations yourapp --empty创建迁移并更新生成的文件：

from django.db import migrations

def export_customer_company(apps, schema_editor):
    Customer = apps.get_model('yourapp', 'Customer')
    Company = apps.get_model('yourapp', 'Company')
    for customer in Customer.objects.all():
        customer._company = Company.objects.get_or_create(name=customer.company)[0]
        customer.save()

def revert_export_customer_company(apps, schema_editor):
    Customer = apps.get_model('yourapp', 'Customer')
    Company = apps.get_model('yourapp', 'Company')
    for customer in Customer.objects.filter(_company__isnull=False):
        customer.company = customer._company.name
        customer.save()

class Migration(migrations.Migration):

    dependencies = [
        ('yourapp', 'xxxx_previous_migration'),  # Note this is auto-generated by django
    ]

    operations = [
        migrations.RunPython(export_customer_company, revert_export_customer_company),
    ]

上面的迁移将根据 Company填充您的 Customer._company模型和 Customer.company字段。

现在，您可以删除旧的 Customer.company并重命名 Customer._company。

class Customer(models.Model):
    name = models.CharField(max_length=100)
    age = models.IntegerField()
    company = models.ForeignKey(Company)

最终 manage.py makemigrations和 manage.py migrate。