Merge pull request 'Only vote when all teachers registered' (#94) from only-vote-when-teachers-registered into main

Reviewed-on: mentorenwahl/mentorenwahl#94

docker/backend/src/backend/api/schema/mutation.cr

@@ -143,6 +143,11 @@ module Backend
         def create_vote(context : Context, input : VoteCreateInput) : Vote
           context.student!
 
+          raise "Not enough teachers" if input.teacher_ids.empty?
+          teacher_role_count = Db::User.where(role: Db::UserRole::Teacher.to_s).count.run.as(Int64)
+          raise "Teachers not registered" if teacher_role_count != Db::Teacher.count ||
+                                             teacher_role_count.zero?
+
           skif = context.external.as(Db::Student).skif
           input.teacher_ids.each do |id|
             teacher = Db::Teacher.find(id)