从“多对多”迁移数据到“多对多通过”在django

问题描述

我有一个模型

class Category(models.Model):
    title           = models.CharField(...)
    entry           = models.ManyToManyField(Entry,null=True,blank=True,
                                             related_name='category_entries',
                                             )

我想重构每个关系的附加数据：

That I wish to refactor to have additional data with each relationship:

class Category(models.Model):
    title           = models.CharField(...)
    entry           = models.ManyToManyField(Entry,null=True,blank=True,
                                             related_name='category_entries',
                                             through='CategoryEntry',
                                             )

But south deletes the existing table. How can I preserve the existing m-t-m relationships?

推荐答案

1. 额外的领域，现在。给它一个唯一的约束，以匹配现有的约束，并指定表名称以匹配现有的名称：

1. Create your intermediate model without any extra fields, for now. Give it a unique constraint to match the existing one and specify the table name to match the existing one:

class CategoryEntry(models.Model):
    category = models.ForeignKey(Category)
    entry = models.ForeignKey(Entry)

    class Meta:
        db_table='main_category_entries'   #change main_ to your application
        unique_together = (('category', 'entry'))

运行南方模式迁移

Run the South schema migration.

编辑生成的模式迁移脚本，并注释掉所有的向前和向后的条目，使用现有的交叉表。添加 pass 来完成这些方法。

Edit the generated schema migration script and comment-out all the forwards and backwards entries, since you'll be re-using the existing intersection table. Add pass to complete the methods.

运行迁移。

更新任何现有的代码。如，与正常的多对多字段不同，您不能使用添加，创建或分配来创建关系，所以你需要修改任何现有的应用程序代码，例如

Update any existing code. As it says in https://docs.djangoproject.com/en/dev/topics/db/models/#many-to-many-relationships, "Unlike normal many-to-many fields, you can't use add, create, or assignment to create relationships" so you'll need to modify any existing application code, e.g.

c.entry.add(e)

可能成为：

try:
    categoryentry = c.categoryentry_set.get(entry = e)
except CategoryEntry.DoesNotExist:
    categoryentry = CategoryEntry(category=c, entry=e)
    categoryentry.save()

和：

e.category_entries.add(c)

可能成为：

categoryentry = CategoryEntry(category=c, entry=e)  #set extra fields here
categoryentry.save()

和：

c.entry.remove(e)

可能成为：

categoryentry = c.categoryentry_set.get(entry = e)
categoryentry.delete()

一旦这个初始伪迁移完成，您应该可以将额外的字段添加到 CategoryEntry 并正常创建进一步的迁移。

Once this initial pseudo migration has been done, you should then be able to add the extra fields to the CategoryEntry and create further migrations as normal.

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