Is this the way to validate Django model fields?

Question

As I understand it, when one creates a Django application, data is validated by the form before it\'s inserted into a model instance which is then written to the database.

Answer 1

Firstly, you shouldn't override full_clean as you have done. From the django docs on full_clean:

Model.full_clean(exclude=None)
This method calls Model.clean_fields(), Model.clean(), and Model.validate_unique(), in that order and raises a ValidationError that has a message_dict attribute containing errors from all three stages.

So the full_clean method already calls clean, but by overriding it, you've prevented it calling the other two methods.

Secondly, calling full_clean in the save method is a trade off. Note that full_clean is already called when model forms are validated, e.g. in the Django admin. So if you call full_clean in the save method, then the method will run twice.

It's not usually expected for the save method to raise a validation error, somebody might call save and not catch the resulting error. However, I like that you call full_clean rather than doing the check in the save method itself - this approach allows model forms to catch the problem first.

Finally, your clean method would work, but you can actually handle your example case in the model field itself. Define your CharField as

name = models.CharField(max_length=128)

The blank option will default to False. If the field is blank, a ValidationError will be raised when you run full_clean. Putting default=None in your CharField doesn't do any harm, but it is a bit confusing when you don't actually allow None as a value.

Answer 2

After thinking about Alasdair's answer and doing addtional reading, my sense now is that Django's models weren't designed so as to be validated on a model-only basis as I'm attempting to do. Such validation can be done, but at a cost, and it entails using validation methods in ways they weren't intended for.

Instead, I now believe that any constraints other than those that can be entered directly into the model field declarations (e.g. "unique=True") are supposed to be performed as a part of Form or ModelForm validation. If one wants to guard against entering invalid data into a project's database via any other means (e.g. via the ORM while working within the Python interpreter), then the validation should take place within the database itself. Thus, validation could be implemented on three levels: 1) First, implement all constraints and triggers via DDL in the database; 2) Implement any constraints available to your model fields (e.g. "unique=True"); and 3) Implement all other constraints and validations that mirror your database-level constraints and triggers within your Forms and ModelForms. With this approach, any form validation errors can be re-displayed to the user. And if the programmer is interacting directly with the database via the ORM, he/she would see the database exceptions directly.

Thoughts anyone?

Answer 3

Capturing the pre-save signals on on my models ensured clean will be called automatically.

from django.db.models.signals import pre_save

def validate_model(sender, **kwargs):
    if 'raw' in kwargs and not kwargs['raw']:
        kwargs['instance'].full_clean()

pre_save.connect(validate_model, dispatch_uid='validate_models')