Django迁移时如何设置/提供默认值？ [英] How can I set/provide a default value while django migration?

问题描述

场景：

我有一个模型，客户

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

现在我使用 ForeignKey 关系更新了 company 属性，如下所示，

and now I updated the company attribute witha ForeignKey relationship as below,

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，将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< code >客户实例。

可能吗？我该如何实现呢？

What I need is, when the new migrations applied to the DB,corresponding Company instance must automatically generate and map to the company attribute of Customer instance.

Is that possible? How can I achieve this ?

推荐答案

让我们从您的原始模型开始，然后逐步进行。

Let's start from your original model and do it step by step.

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，以便以后可以还原旧数据。

First you would have to keep the original field and create a new one, to be able to restore the old data afterwards.

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 <创建第一次迁移/ code>。然后，您将必须创建数据迁移。使用 manage.py makemigrations yourapp --empty 创建迁移并更新生成的文件：

Now you can create a first migration with manage.py makemigrations. Then you will have to create a data migration. Create the migration using manage.py makemigrations yourapp --empty and update the generated file:

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),
    ]

以上迁移将填充您的公司模型和客户。 _company 字段，根据 Customer.company 。

The above migration will populate your Company model and Customer._company field according to Customer.company.

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

Now you can drop the old Customer.company and rename 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迁移。

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