Django Model Validation

I'm really excited about model validation, in Django circa 1.2. It's going to save me from the disastrous hack of ModelForms I've used so far. One wants something to validate data before saving it, since validating it by saving it is apparently a Bad Idea, since you may have already made changes to the database by the time the error is thrown.

I wanted this done automatically, and Django only had form validation until 1.2, so I hacked forms into some sort of all-purpose wrapper for models. I've since learned to program Django like an adult, and model validation came around just in time anyway. But there's a problem I have with it.

First I should point out that there's a few different things that get validated, but the two I'll point out are A) checking that required fields are set and B) whatever I put into the clean() function. Judging by some errors I've gotten while debugging/testing, Django seems to be checking B before A.

Now, when I use a ModelForm, sometimes I want to use the commit=False option when I save. This returns a model that hasn't been committed to the database. Sometimes there's extra data I want to add to the model that the form didn't supply. Sometimes that data is in fact necessary for the model to be valid. So clearly Django shouldn't check A, and it doesn't. Here's the funny thing though: it does check B. Why would it do that? I can understand checking when I call is_valid(), I can control when that's called, making sure I added everything first.

So far the consequences of checking B early have been trying to access members that haven't yet been set, in my clean() function. So in clean() I just check for those items, and just let it pass if they don't exist. I figure, yeah, it'll pass certain tests when it shouldn't. But if it's really running through validation (not just B as in the save (commit=False) case) that means it'll catch the missing members on the A pass anyway.

Maybe I got something wrong, but I thought it was a weird design decision.