On Tue, Aug 18, 2009 at 8:26 PM, ringemup <ringe...@gmail.com> wrote:
>
> I have accounts that can have multiple aliases, but each account must
> have a primary alias. I can think of two ways to institute this, but
> they both have problems:
>
> 1) reference the primary alias from the account:
>
> class Account(models.Model):
> ...
> primary_alias = models.OneToOneField('Alias',
> related_name='accout_if_primary')
>
> class Alias(models.Model):
> name = models.CharField(max_length=50, primary_key=True)
> account = models.ForeignKey(Account)
>
> The trouble with this approach is that basically you can't create an
> account without an alias, and you can't create an alias without an
> account because of what amount to circular references, so you
> essentially can't add any data.
>
> 2) Assign primary status to the alias:
>
> class Account(models.Model):
> ...
>
> class Alias(models.Model):
> name = models.CharField(max_length=50, primary_key=True)
> account = models.ForeignKey(Account)
> is_primary = models.BooleanField(default=False)
>
> The trouble here is that it is a real pain to enforce that each
> account has a primary alias (in fact you have to initially create an
> account with no aliases and then create aliases and add them to it).
> Additionally, enforcing a limit on the number of aliases is
> problematic. Finally, even if you do enforce these constraints
> programmatically, it doesn't seem to be feasible to relay error
> messages to contrib.admin.
>
> Has anyone else encountered this design problem, and how did you go
> about addressing it?
>
I have experienced this situation a couple of times and I would recommend
the second option you discussed. Circular referenced like your first option
can become very problematic and are not recommended from a database design
perspective.
What I would recommend is to create an Alias record automatically when a new
Account is created. You can do this in the save of the Account model or with
signals.
http://docs.djangoproject.com/en/dev/ref/models/instances/
http://docs.djangoproject.com/en/dev/ref/signals/
Then in the save of the Alias you can manage the primary flag. I just check
to see if the current record being saved has primary set, if so then I reset
all others for (in your case) the account to not primary. The only other
case is if the current alias isn't set as primary, check to see if there are
any primary aliases yet and if not automatically set the current one as
primary.
Ya, it's a little tricky but it's worth not having the headache of the
circular reference.
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