Handle apps with migrations disabled via MIGRATION_MODULES (#24)

Fixes #23.

HISTORY.rst

@@ -2,6 +2,12 @@
 History
 =======
 
+* Handle apps with whose migrations have been disabled by mapping them to
+  ``None`` in the ``MIGRATION_MODULES`` setting.
+
+  Thanks to Helmut for the report in `Issue #23
+  <https://github.com/adamchainz/django-linear-migrations/issues/23>`__.
+
 1.2.0 (2020-12-14)
 ------------------
 

src/django_linear_migrations/apps.py

@@ -39,14 +39,17 @@ def __init__(self, app_label, do_reload=False):
         self.migrations_module_name, _explicit = MigrationLoader.migrations_module(
             app_label
         )
-        try:
-            self.migrations_module = import_module(self.migrations_module_name)
-        except ModuleNotFoundError:
-            # Unmigrated app
+        if self.migrations_module_name is None:
             self.migrations_module = None
         else:
-            if do_reload:
-                reload(self.migrations_module)
+            try:
+                self.migrations_module = import_module(self.migrations_module_name)
+            except ModuleNotFoundError:
+                # Unmigrated app
+                self.migrations_module = None
+            else:
+                if do_reload:
+                    reload(self.migrations_module)
 
     @property
     def has_migrations(self):

tests/test_create_max_migration_files.py

@@ -34,6 +34,15 @@ def call_command(self, *args, **kwargs):
             returncode = exc.code
         return out.getvalue(), err.getvalue(), returncode
 
+    def test_success_migrations_disabledi(self):
+        self.migrations_dir.rmdir()
+        with override_settings(MIGRATION_MODULES={"testapp": None}):
+            out, err, returncode = self.call_command()
+
+        assert out == "No max_migration.txt files need creating.\n"
+        assert err == ""
+        assert returncode == 0
+
     def test_success_no_migrations_dir(self):
         self.migrations_dir.rmdir()