Add soft validation of member codes to training request update form

[elichad]

Fixes #2550.

The implementation is very similar to #2549, but allows the actual reason for failure to be shown (rather than the list of possible reasons).

While testing I discovered some CharField and TextField fields on TrainingRequest which had blank=True, null=True, so I changed these to have default="" instead in line with best practices.

[pbanaszkiewicz]

Looks good, but I strongly suggest moving member_code_valid to exception-based approach.

[pbanaszkiewicz]

Thanks for these changes.

[pbanaszkiewicz]

What causes this? Can you use a rebased codebase or alter migration 0269_alter_trainingrequest_nonprofit_teaching_experience (it's okay before we release to prod, and even safer if it wasn't released on test-amy)?

[elichad]

I squashed two uncommitted migrations to make this one, and this line resulted from that. I'll delete it.

[pbanaszkiewicz]

You could use custom exceptions instead of returning a flag and text.
For example:

class MemberCodeValidationError(ValueError):  # or inherit from ValidationError
  pass

...

    def member_code_valid(self, code: str) -> bool:
        """Returns True if the code matches an active Membership with available seats,
        including a grace period of 90 days before and after the Membership dates.
        If there is no match, throws an exception with detailed error.
        """
        try:
            # find relevant membership - may raise Membership.DoesNotExist
            membership = Membership.objects.get(registration_code=code)
        except Membership.DoesNotExist as exc:
            raise MemberCodeValidationError(f'No membership found for code "{code}".') from exc

        ...
        return True

[pbanaszkiewicz]

I'd consider making this a function receiving code and timestamp (to be used instead of self.instance.created_at) and moving outside the class - as a util function. I'd also add unit tests for it.