Make professor aliases unique

[nsandler1]

No description provided.

[tybug]

we should also change merging professors to check for duplicate name in alias, instead of leaving it to throw a 500 error. Not sure what the best approach here is. For now we can probably just refuse to merge a professor if it would create a duplicate alias, and we can manually fix things up in the django admin panel if necessary? Depends how often this comes up.

[nsandler1]

Good point, though I don't think we need to abort the merge. Since the problem alias already exists as an alias of the merge target, we can just delete it from the merge subject's aliases before the aliases are merged.

[nsandler1]

Disregard my last comment, I realized we already have a check in place to (almost) prevent your concern from happening (in admin.py):

170  ProfessorAlias.objects.filter(professor=merge_subject).update(professor=merge_target)
171
172  aliases = ProfessorAlias.objects.filter(alias=merge_subject.name, professor=merge_target)
173  if not aliases.exists():
174       ProfessorAlias(alias=merge_subject.name, professor=merge_target).save()

I should probably change the aliases query to only filter on the alias, not the alias and the professor. Other than that, this fragment should prevent any integrity violations. To clarify, even the initial bulk update won't cause any issues because we currently don't have any duplicate aliases (I checked) and the conditional checks for existing aliases before creating a new one.

EDIT: #84 changes line 172 to aliases = ProfessorAlias.objects.filter(alias=merge_subject.name) which does prevent your concern from happening

[tybug]

yep, not creating any alias at all in a case like this sounds like a good approach.