django 迁移时如何设置/提供默认值？

Scenario:
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)

现在我更新了company属性带有一个ForeignKey关系如下，

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，当新的迁移应用于数据库时，对应Company实例必须自动生成并映射到company的属性Customer实例。

那可能吗？我怎样才能做到这一点？

让我们从您的原始模型开始，一步一步地进行。

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，以便之后能够恢复旧数据。

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

现在您可以使用以下命令创建第一个迁移manage.py makemigrations。然后你必须创建一个数据迁移 https://docs.djangoproject.com/en/stable/howto/writing-migrations/。使用创建迁移manage.py makemigrations yourapp --empty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),
    ]

上述迁移将填充您的Company模型和Customer._company字段根据Customer.company.

现在你可以放弃旧的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)

Final manage.py makemigrations and manage.py migrate.