Help a Python newbie with a Django model inheritance problem

Question

I'm working on my first real Django project after years of PHP programming, and I am running into a problem with my models. First, I noticed that I was copying and pasting code between the models, and being a diligent OO programmer I decided to make a parent class that the other models could inherit from:

class Common(model.Model):
    name = models.CharField(max_length=255)
    date_created  = models.DateTimeField(auto_now_add=True)
    date_modified = models.DateTimeField(auto_now=True)

    def __unicode__(self):
        return self.name

    class Meta:
        abstract=True

So far so good. Now all my other models extend "Common" and have names and dates like I want. However, I have a class for "Categories" were the name has to be unique. I assume there should be a relatively simple way for me to access the name attribute from Common and make it unique. However, the different methods I have tried to use have all failed. For example:

class Category(Common):
    def __init__(self, *args, **kwargs):
        self.name.unique=True

Causes the Django admin page to spit up the error "Caught an exception while rendering: 'Category' object has no attribute 'name'

Can someone point me in the right direction?

Answer 1

You have a small mistake in your Common class

class Common(model.Model):
    self.name = models.CharField(max_length=255) 

should be

class Common(model.Model):
    name = models.CharField(max_length=255)

Answer 2

No, Django doesn't allow that.

See the docs: http://docs.djangoproject.com/en/1.1/topics/db/models/#field-name-hiding-is-not-permitted

Also answered in other questions like: http://stackoverflow.com/questions/2344751/in-django-model-inheritance-does-it-allow-you-to-override-a-parent-models-at

Answer 3

Try using Meta.unique_together to force it into its own unique index. Failing that, it's probably easiest to create two separate abstract classes, one with the field unique and one not.

Answer 4

Note that UNIQUE constraint in fact has nothing to do with Django, so you're free to add it in your database table. You can also use post-syncdb hook for that purpose.