Fix create-max-migration-files for apps without migrations

Fixes #13.

HISTORY.rst

@@ -2,12 +2,21 @@
 History
 =======
 
+* Fix ``create-max-migration-files`` for apps without migrations folders or
+  files.
+
+  Thanks to Ferran Jovell for the report in `Issue #13
+  <https://github.com/adamchainz/django-linear-migrations/issues/13>`__.
+
 1.0.1 (2020-12-11)
 ------------------
 
 * Move initial ``max_migration.txt`` file creation into a separate management
   command, ``create-max-migration-files``.
 
+  Thanks to Ferran Jovell for the report in `Issue #11
+  <https://github.com/adamchainz/django-linear-migrations/issues/13>`__.
+
 1.0.0 (2020-12-10)
 ------------------
 

src/django_linear_migrations/management/commands/create-max-migration-files.py

@@ -43,8 +43,11 @@ def handle(self, *app_labels, **options):
                 continue
 
             migration_details = MigrationDetails(app_config.label)
+            if not migration_details.has_migrations:
+                continue
+
             max_migration_txt = migration_details.dir / "max_migration.txt"
-            if not max_migration_txt.exists():
+            if not max_migration_txt.exists() and len(migration_details.names) > 0:
                 max_migration_name = max(migration_details.names)
                 max_migration_txt.write_text(max_migration_name + "\n")
                 self.stdout.write(

tests/test_create_max_migration_files.py

@@ -34,6 +34,31 @@ def call_command(self, *args, **kwargs):
             returncode = exc.code
         return out.getvalue(), err.getvalue(), returncode
 
+    def test_success_no_migrations_dir(self):
+        self.migrations_dir.rmdir()
+
+        out, err, returncode = self.call_command()
+
+        assert out == "No max_migration.txt files need creating.\n"
+        assert err == ""
+        assert returncode == 0
+
+    def test_success_empty_migrations_dir(self):
+        out, err, returncode = self.call_command()
+
+        assert out == "No max_migration.txt files need creating.\n"
+        assert err == ""
+        assert returncode == 0
+
+    def test_success_only_init(self):
+        (self.migrations_dir / "__init__.py").touch()
+
+        out, err, returncode = self.call_command()
+
+        assert out == "No max_migration.txt files need creating.\n"
+        assert err == ""
+        assert returncode == 0
+
     def test_success(self):
         (self.migrations_dir / "__init__.py").touch()
         (self.migrations_dir / "0001_initial.py").touch()