Question

我有两个类，其中一个是另一个类的后代，我想让它们成为来自同一基类的兄弟类。

在：

from django.db import models

class A(models.Model):
    name = models.CharField(max_length=10)

class B(models.Model):
    title = models.CharField(max_length=10)

后：

from django.db import models

class Base(models.Model):
    name = models.CharField(max_length=10)

class A(Base):
    pass

class B(Base):
    title = models.CharField(max_length=10)

当我生成架构迁移时，这是输出，包括我对问题的回答：

+ Added model basetest.Base
? The field 'B.a_ptr' does not have a default specified, yet is NOT NULL.
? Since you are removing this field, you MUST specify a default
? value to use for existing rows. Would you like to:
?  1. Quit now, and add a default to the field in models.py
?  2. Specify a one-off value to use for existing columns now
?  3. Disable the backwards migration by raising an exception.
? Please select a choice: 3
- Deleted field a_ptr on basetest.B
? The field 'B.base_ptr' does not have a default specified, yet is NOT NULL.
? Since you are adding this field, you MUST specify a default
? value to use for existing rows. Would you like to:
?  1. Quit now, and add a default to the field in models.py
?  2. Specify a one-off value to use for existing columns now
? Please select a choice: 2
? Please enter Python code for your one-off default value.
? The datetime module is available, so you can do e.g. datetime.date.today()
>>> 37
+ Added field base_ptr on basetest.B
? The field 'A.id' does not have a default specified, yet is NOT NULL.
? Since you are removing this field, you MUST specify a default
? value to use for existing rows. Would you like to:
?  1. Quit now, and add a default to the field in models.py
?  2. Specify a one-off value to use for existing columns now
?  3. Disable the backwards migration by raising an exception.
? Please select a choice: 3
- Deleted field id on basetest.A
? The field 'A.name' does not have a default specified, yet is NOT NULL.
? Since you are removing this field, you MUST specify a default
? value to use for existing rows. Would you like to:
?  1. Quit now, and add a default to the field in models.py
?  2. Specify a one-off value to use for existing columns now
?  3. Disable the backwards migration by raising an exception.
? Please select a choice: 3
- Deleted field name on basetest.A
? The field 'A.base_ptr' does not have a default specified, yet is NOT NULL.
? Since you are adding this field, you MUST specify a default
? value to use for existing rows. Would you like to:
?  1. Quit now, and add a default to the field in models.py
?  2. Specify a one-off value to use for existing columns now
? Please select a choice: 2
? Please enter Python code for your one-off default value.
? The datetime module is available, so you can do e.g. datetime.date.today()
>>> 73
+ Added field base_ptr on basetest.A
Created 0002_auto__add_base__del_field_b_a_ptr__add_field_b_base_ptr__del_field_a_i.py. You can now apply this migration with: ./manage.py migrate basetest

我不知道如何回答有关B.base_ptr和A.base_ptr的默认值的问题。我给出的任何常量都会导致迁移在运行时失败，输出结果为：

FATAL ERROR - The following SQL query failed: CREATE TABLE "_south_new_basetest_a" ()
The error was: near ")": syntax error
RuntimeError: Cannot reverse this migration. 'B.a_ptr' and its values cannot be restored.

这是我顺便使用sqlite3的结果。使用Postgres就是这样的：

FATAL ERROR - The following SQL query failed: ALTER TABLE "basetest_a" ADD COLUMN "base_ptr_id" integer NOT NULL PRIMARY KEY DEFAULT 73;
The error was: could not create unique index "basetest_a_pkey"
DETAIL:  Key (base_ptr_id)=(73) is duplicated.

Error in migration: basetest:0002_auto__add_base__del_field_b_a_ptr__add_field_b_base_ptr__del_field_a_i
IntegrityError: could not create unique index "basetest_a_pkey"
DETAIL:  Key (base_ptr_id)=(73) is duplicated.

我应该使用什么值来使base_ptr进行此迁移？谢谢！

Answer 1

如果base无法自行实例化，您可以使用abstract = True道具轻松解决问题{.1}}。

示例代码：

class Meta

Answer 2

您可以分阶段完成此任务。

阶段1：在代码中创建“基础”模型。在A和B模型上，将base_ptr作为可空的FK添加到Base（名称base_ptr通过小写类名Base来生成，相应地调整您的名称）。在新列上指定db_column='base_ptr'，这样就不会添加_id后缀。不要改变父母身份：保持B为A和A之前的孩子（Base还没有子课程）。添加迁移以进行相应的数据库更改，然后运行它。

阶段2：创建数据迁移，复制相关数据。您应该将所有A数据复制到Base，删除多余的A记录（提供B个实例的记录），以及剩余记录（A并B）将ID复制到base_ptr。请注意，子类B使用两个表 - 其id字段来自A的表，并且在其自己的表中有一个字段a_ptr，它是一个FK到A - 因此，如果您将值从a_ptr复制到base_ptr，则更新操作会更有效。确保在复制到Base表后复制到base_ptr，这样就不会违反FK约束。

阶段3：现在再次更改模型 - 删除显式base_ptr FK并按照您喜欢的方式更改父项，并创建第三次迁移（自动模式迁移）。请注意，将父级设置为Base隐式定义了一个不可为空的base_ptr字段，因此对于base_ptr字段，您只是将可空字段更改为不可为空，而不是需要默认值。

我们仍然会要求您提供a_ptr的默认值 - 从父B更改为A时删除的从A到Base的隐式FK a_ptr;向后迁移需要默认值。您可以执行一些无法进行向后迁移的操作，或者，如果您确实要支持它，请向B添加一个明确的可为空base_ptr，就像之前使用的{{1}}列一样。然后可以在第四次迁移中删除此可以为空的列。

Answer 3

或者您可以通过在元数据中添加以下内容来将类指定为抽象类：

class Meta:
    abstract =True

在使用South迁移重新授权的类时，我对_ptr使用什么值？

3 个答案: