Add exception unit tests.

Author: WaVEV

tests/model_fields_/test_objectidfield.py

@@ -1,4 +1,5 @@
 from bson import ObjectId
+from django.core import exceptions
 from django.test import SimpleTestCase
 
 from django_mongodb.fields import ObjectIdField
@@ -19,5 +20,15 @@ def test_get_internal_type(self):
     def test_to_python(self):
         f = ObjectIdField()
         expected = ObjectId("1" * 24)
-        self.assertEqual(f.to_python("1" * 24), expected)
-        self.assertEqual(f.to_python(expected), expected)
+        for value in ["1" * 24, ObjectId("1" * 24)]:
+            with self.subTest(value=value):
+                self.assertEqual(f.to_python(value), expected)
+        with self.subTest(value=None):
+            self.assertIsNone(f.to_python(None))
+
+    def test_to_python_invalid_value(self):
+        for invalid_value in [3, "None", {}, []]:
+            with self.subTest(invalid_value=invalid_value):
+                msg = f"['“{invalid_value}” value must be an Object Id.']"
+                with self.assertRaisesMessage(exceptions.ValidationError, msg):
+                    ObjectIdField().to_python(invalid_value)