Allow multiple manufacturers per class (#47)

* Allow multiple manufacturers per class

* Fix REST API serializer

ram/roster/migrations/0033_rename_manufacturer_rollingclass_manufacturer_old.py

@@ -0,0 +1,45 @@
+# Generated by Django 5.1.4 on 2025-01-20 21:25
+
+from django.db import migrations, models
+
+
+def manufacturer_to_many(apps, schema_editor):
+    rolling_class = apps.get_model("roster", "RollingClass")
+    for row in rolling_class.objects.all():
+        manufacturer = row.manufacturer_old
+        if manufacturer:
+            row.manufacturer.add(manufacturer)
+        row.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("metadata", "0022_decoderdocument_creation_time_and_more"),
+        ("roster", "0032_rollingstockdocument_creation_time_and_more"),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name="rollingclass",
+            old_name="manufacturer",
+            new_name="manufacturer_old",
+        ),
+        migrations.AddField(
+            model_name="rollingclass",
+            name="manufacturer",
+            field=models.ManyToManyField(
+                blank=True,
+                limit_choices_to={"category": "real"},
+                to="metadata.manufacturer",
+            ),
+        ),
+        migrations.RunPython(
+            manufacturer_to_many,
+            reverse_code=migrations.RunPython.noop
+        ),
+        migrations.RemoveField(
+            model_name="rollingclass",
+            name="manufacturer_old",
+        ),
+    ]