Disallow naming field _state in Django

friendly_states/django.py

@@ -107,6 +107,11 @@ def deconstruct(self):
         return name, path, (self.machine,), kwargs
 
     def contribute_to_class(self, cls, name, *args, **kwargs):
+        if name == "_state":
+            raise ValueError(
+                "_state is an internal attribute used by Django "
+                "and should not be the name of a field."
+            )
         super().contribute_to_class(cls, name, *args, **kwargs)
         self.machine.attr_name = self.attname
 

tests/test_django.py

@@ -1,6 +1,6 @@
 import pytest
 from django.core.exceptions import ValidationError
-from django.db import IntegrityError
+from django.db import IntegrityError, models
 from django.db.transaction import atomic
 
 from friendly_states.core import AttributeState
@@ -172,3 +172,14 @@ class Machine(DjangoState):
                   r"after declaring all states \(subclasses\)."
     ):
         StateField(Machine)
+
+
+def test_underscore_state():
+    with pytest.raises(ValueError):
+        class Model(models.Model):
+            class Meta:
+                app_label = "myapp"
+
+            _state = StateField(TrafficLightMachine)
+
+        str(Model)