rebase_migration: Un apply and rebase migrations in case migrations are applied.

Previously, while rebasing migrations, if the migration to be
rebased is already applied, it would throw out an error asking
to un apply the migration before rebasing.

This commit un applies the migrations and rebases them when
the migrations to be rebased are already applied. This is done
by first creating a --merge migration, un applying the previous
migrations, removing the --merge migration, and then rebasing
the existing migrations.

Fixes #184

Author: roanster007

src/django_linear_migrations/management/commands/rebase_migration.py

@@ -93,11 +93,16 @@ def rebase_migration(
             )
 
         if migration_applied(app_label, rebased_migration_name):
-            raise CommandError(
-                f"Detected {rebased_migration_name} as the rebased migration,"
-                + " but it is applied to the local database. Undo the rebase,"
-                + " reverse the migration, and try again."
-            )
+            try:
+                unapply_migrations(
+                    app_label, rebased_migration_name, self.last_migration_name
+                )
+            except (FileNotFoundError, subprocess.SubprocessError):
+                raise CommandError(
+                    f"Detected {rebased_migration_name} as the rebased migration,"
+                    + " but it is applied to the local database. Undo the rebase,"
+                    + " reverse the migration, and try again."
+                )
 
         content = rebased_migration_path.read_text()
         split_result = re.split(
@@ -272,3 +277,54 @@ def migration_applied(app_label: str, migration_name: str) -> bool:
             # django_migrations table does not exist -> no migrations applied
             pass
     return False
+
+
+def unapply_migrations(
+    app_label: str, first_rebase_migration: str, last_merged_migration: str
+) -> None:  # pragma: no cover
+    first_rebase_migration_number, _merged_rest = first_rebase_migration.split("_", 1)
+    last_merged_number, _merged_rest = last_merged_migration.split("_", 1)
+    migration_details = MigrationDetails(app_label)
+
+    subprocess.run(
+        [
+            "./manage.py",
+            "makemigrations",
+            "--merge",
+            "--noinput",
+            "--skip-checks",
+            f"{app_label}",
+        ],
+        check=True,
+    )
+
+    last_migration_to_be_applied = None
+    merge_migration_name = None
+
+    for migration_name in migration_details.names:
+        if migration_name.startswith(
+            f"{int(first_rebase_migration_number) - 1}".zfill(4)
+        ):
+            last_migration_to_be_applied = migration_name
+
+        elif migration_name.startswith(f"{int(last_merged_number) + 1}".zfill(4)):
+            merge_migration_name = migration_name
+
+    assert last_migration_to_be_applied is not None and merge_migration_name is not None
+    merge_migration_path = migration_details.dir / f"{merge_migration_name}.py"
+
+    subprocess.run(
+        [
+            "./manage.py",
+            "migrate",
+            "--skip-checks",
+            f"{app_label}",
+            last_migration_to_be_applied,
+        ],
+        check=True,
+    )
+
+    subprocess.run(
+        ["rm", f"{merge_migration_path}"],
+        check=True,
+    )