Adding a "through" table to django field and migrating with South?
Seems like this should be "easy" or at least documented somewhere, I just cant find it.
Lets say I have a model:
class A(models.Model):
users = models.ManyToMany('auth.User', blank=True)
Now I want to migrate to have a through
table to add fields to the ManyToMany relation...
class AUsers(models.Model):
user = models.ForeignKey('auth.User')
a = models.ForeignKey('A')
new_field = models.BooleanField()
class A(models.Model):
users = models.ManyToMany('auth.User', blank=True, through='AUsers')
开发者_运维问答Then I do:
% ./manage.py schemamigration app --auto
Not totally surprising, it tells me it is going to drop the original auto-created through table and create a new one for AUsers
. What's the best practice at this point? Is there a decent way to migrate to the new through
table? Do I use db_table
in Meta? Do I just not use the through=...
right away... then do a schemamigration --auto
, then a datamigration
to copy the current table (somehow, not sure...) and then add the through
relation and let it kill the table?
What's the trick here? Is this really that hard?
You should be able to do this pretty easily.
First of all, make sure that the manual through table that you are creating has the same table name in the database as the one Django originally created automatically.
So, first, let's consider a manual through model before your change:
class AUsers(models.Model):
user = models.ForeignKey('auth.User')
a = models.ForeignKey('A')
class Meta:
db_table = 'appname_a_user'
That should be functionally (almost) identical to the ManyToManyField
you used to have. Actually, you could make an empty migration and apply it, and then use --auto for your changes (but don't).
Now, add your field like you did in your sample code above, and then run ./manage.py schemamigration appname manual_through_table --empty
. That will give you an empty migration named ####_manual_through_table.py
.
In the migration itself, there will be a forwards
and backwards
method. Each one needs to be one line each:
def forwards(self, orm):
db.add_column('appname_a_user', 'new_field', self.gf('django.db.models.fields.BooleanField')(default=False))
def backwards(self, orm):
db.delete_column('appname_a_user', 'new_field')
That should get you what you are after.
If anyone comes across this question when trying to do the same thing with the moderns migration framework, here are the steps:
- Create a new model class that exactly matches the built-in through table
- Use the Meta class to set the table name to match the existing table
- Generate a migration, which will create the new table and set it as the through for the field.
- Without running that migration, edit it to wrap it in a
migrations. SeparateDatabaseAndState
migration, where the auto-generated steps are in thestate_operations
field and the database operations are empty. - Modify your through table, as required, making sure to generate new migrations as normal.
As mentioned in a comment, the first step may be simplified using db.rename_table
as described here, which gives this through model:
class AUsers(models.Model):
user = models.ForeignKey('auth.User')
a = models.ForeignKey('A')
class Meta:
unique_together = (('user', 'a'),)
Then, create a migration with --auto (this way you'll have the names of the DB tables visible), and replace the content with:
class Migration(SchemaMigration):
def forwards(self, orm):
db.rename_table('appname_a_user', 'appname_auser')
def backwards(self, orm):
db.rename_table('appname_auser','appname_a_user')
I just applied it in my project without issues.
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