validate move before migration

src/documents/migrations/1012_fix_archive_files.py

@@ -74,12 +74,18 @@ def move_old_to_new_locations(apps, schema_editor):
     # check for documents that have incorrect archive versions
     for doc in Document.objects.filter(archive_checksum__isnull=False):
         old_path = archive_path_old(doc)
+        new_path = archive_path_new(doc)
 
         if not os.path.isfile(old_path):
             raise ValueError(
                 f"Archived document of {doc.filename} does not exist at: "
                 f"{old_path}")
 
+        if old_path != new_path and os.path.isfile(new_path):
+            raise ValueError(
+                f"Need to move {old_path} to {new_path}, but target file "
+                f"already exists")
+
         if old_path in old_archive_path_to_id:
             affected_document_ids.add(doc.id)
             affected_document_ids.add(old_archive_path_to_id[old_path])

src/documents/tests/test_migration_archive_files.py

@@ -141,6 +141,8 @@ class TestMigrateArchiveFiles(DirectoriesMixin, TestMigrations):
                     archive_checksum = hashlib.md5(f.read()).hexdigest()
                 self.assertEqual(archive_checksum, doc.archive_checksum)
 
+        self.assertEqual(Document.objects.filter(archive_checksum__isnull=False).count(), 4)
+
         # this will raise errors when any inconsistencies remain after migration
         sanity_check()