fix: Show helfpul error when invalid target data model is selected

traffic_control/models/common.py

@@ -169,6 +169,7 @@ def traffic_sign_type(self):
 
     def clean(self):
         self.validate_target_model_content_schema()
+        self.validate_change_target_model(self.target_model, raise_exception=True)
 
     def validate_target_model_content_schema(self):
         target_models_with_content_schema = (DeviceTypeTargetModel.ADDITIONAL_SIGN,)

traffic_control/tests/test_traffic_control_device_type_admin.py

@@ -0,0 +1,48 @@
+import pytest
+from django import forms
+
+from traffic_control.enums import DeviceTypeTargetModel
+from traffic_control.models.common import TrafficControlDeviceType
+from traffic_control.tests.factories import AdditionalSignRealFactory, TrafficControlDeviceTypeFactory
+
+
+class TrafficControlDeviceTypeForm(forms.ModelForm):
+    class Meta:
+        model = TrafficControlDeviceType
+        fields = "__all__"
+
+
+@pytest.mark.parametrize("create_related_object, expect_valid", ((False, True), (True, False)))
+@pytest.mark.django_db
+def test__traffic_control_device_type_admin__change_target_model(create_related_object, expect_valid):
+    """
+    The traffic control device type form prevents an editor from altering the target
+    model of a traffic control device type that is currently in use. The error message
+    will be informative.
+
+    """
+    data = {
+        "code": "D123",
+        "icon": "d123.svg",
+        "description": "A test device type",
+        "legacy_code": "123D",
+        "legacy_description": "A legacy description",
+        "target_model": DeviceTypeTargetModel.ADDITIONAL_SIGN,
+    }
+    dt = TrafficControlDeviceTypeFactory(**data)
+    data["target_model"] = DeviceTypeTargetModel.TRAFFIC_SIGN
+
+    if create_related_object is True:
+        AdditionalSignRealFactory(device_type=dt)
+    else:
+        pass
+
+    form = TrafficControlDeviceTypeForm(data, instance=dt)
+    if expect_valid:
+        assert form.is_valid() is True
+        form.save()
+    else:
+        assert form.is_valid() is False
+        with pytest.raises(ValueError):
+            form.save()
+        assert "devices related to this device type instance will become invalid" in form.errors["__all__"][0]