Pre-validate uniqueness.

[tomchristie]

Eg. If username is a unique field, then User.objects.create(username=...) should ideally:

• Run inside a transaction.
• Run a query to check no existing username already exists. Raise an exception otherwise.
• Perform the insert.

This is far more helpful than raising an integrity error.

[florimondmanca]

I'm interested in working on this!

Which type of exception should be raised? A new UniqueFieldExists exception? A typesystem.ValidationError?

Also, could you explain why the transaction is required? 😄 Can't it be enough to execute the various validation queries and then perform the insert?

[tomchristie]

A typesystem.ValidationError yup. At least for now.

Also, could you explain why the transaction is required?

Let's not worry about that at the moment. But yeah, ideally you'd want that, otherwise there's a race condition between "We've checked uniqueness" and "we've performed the insert". If you get caught by that race, then you can end up having an DB error raised, rather than a nice validation error.

[tomchristie]

I mean Django doesn't cover it I think, nor REST framework, so it's not really a big deal, and the integrity of the data is still safe, so it's really not a big deal in practice.

[florimondmanca]

Yes, I see. I also suppose we're safe since the data integrity is ensured by the database anyway (at least for MySQL and PostgreSQL).