django：“ unique = True”会阻止“ IntegrityError”吗？

Question

I realized that when I have a simple model:

class Category(models.Model):
    name  = models.CharField(max_length=128, unique=True)
    views = models.IntegerField(default=0)
    likes = models.IntegerField(default=0)

the modifier unique=True has the effect that when I add a category to the database (via an HTML form) which has a name of an instance already present in the database, the application will not crash. Instead, I get access to form.errors which I can print to the page or to the terminal. If I omit unique=True and I try to add a category to the database with a name that already exists, I get an IntegrityError and my application crashes.

So it seems to me that defining unique=True is pretty important for the behaviour of the application. I guess then there must be other model attributes which are equally importa

In the django documentation, where can I read about this and if there are other such attributes?

Answer 1

First of all, you created the model using the unique constraint, so the database already has the unique constraint.Something like (for MySQL):

CONSTRAINT myapp_category UNIQUE (name)

Now, you are probably using a ModelForm , which picks this attribute unique=True , and applies that validation. So, you can catch the errors via form.errors and handle it gracefully.

When you remove the attribute, the database still has the constraint, but the ModelForm does not perform that validation for you, hence the IntegrityError

So, to answer your question - unique=True does help prevent raising of an exception when you use it in the context of a ModelForm , or you would have to handle it yourself to make it fail gracefully.

To remove the unique constraint from the database, you would have to use a migration tool like south

Answer 2

My guess is that you have a database model which has a UNIQUE constraint on the name column. If you try to insert two rows with the same name, then the database (via the database driver) will throw an IntegrityError . This happens completely independent of django.

django then has a model how to build forms. If you omit the unique=True , django assumes that the column in the database isn't unique and doesn't add the necessary checks in the form validation step.

So you basically have a config mismatch between your form and your database.

Solutions:

• Make sure the constraints for forms and database columns are always the same
• Write your own, custom validation rules (see the django documentation).

Alternatively: Write code that gets unique constraints from the database and then, when you create a form, check that the necessary attributes are set to avoid this error.