fix(migration): drop more tables in w.a. for inconsistent 21→22

src/Model/Migration.hs

@@ -530,6 +530,12 @@ customMigrations = Map.fromListWith (>>)
           | oldVersion ->
             -- Major changes happend to the structure of exams without appropriate
             -- migration, try to remedy that here
+            tableDropEmpty "exam_part_corrector"
+            tableDropEmpty "exam_corrector"
+            tableDropEmpty "exam_result"
+            tableDropEmpty "exam_registration"
+            tableDropEmpty "exam_occurrence"
+            tableDropEmpty "exam_part"
             tableDropEmpty "exam"
           | otherwise ->
             [executeQQ|
@@ -570,7 +576,7 @@ tableIsEmpty table = do
   return $ unSingle rows == (0 :: Int64)
 
 tableDropEmpty :: MonadIO m => Text -> ReaderT SqlBackend m ()
-tableDropEmpty table = do
+tableDropEmpty table = whenM (tableExists table) $ do
   isEmpty <- tableIsEmpty table
   if
     | isEmpty   -> rawExecute [st|DROP TABLE "#{table}" CASCADE|] []