Fixed #26743 -- Fixed UnboundLocalError crash when deserializing m2m fields and value isn't iterable.

django/core/serializers/base.py

@@ -282,9 +282,13 @@ def deserialize_m2m_values(field, field_value, using, handle_forward_references)
         def m2m_convert(v):
             return model._meta.pk.to_python(v)
 
+    try:
+        pks_iter = iter(field_value)
+    except TypeError as e:
+        raise M2MDeserializationError(e, field_value)
     try:
         values = []
-        for pk in field_value:
+        for pk in pks_iter:
             values.append(m2m_convert(pk))
         return values
     except Exception as e:

tests/serializers/test_json.py

@@ -252,6 +252,20 @@ class JsonSerializerTestCase(SerializersTestBase, TestCase):
             for obj in serializers.deserialize('json', test_string, ignore=False):
                 obj.save()
 
+    def test_helpful_error_message_for_many2many_not_iterable(self):
+        """
+        Not iterable many-to-many field value throws a helpful error message.
+        """
+        test_string = """[{
+            "pk": 1,
+            "model": "serializers.m2mdata",
+            "fields": {"data": null}
+        }]"""
+
+        expected = "(serializers.m2mdata:pk=1) field_value was 'None'"
+        with self.assertRaisesMessage(DeserializationError, expected):
+            next(serializers.deserialize('json', test_string, ignore=False))
+
 
 class JsonSerializerTransactionTestCase(SerializersTransactionTestBase, TransactionTestCase):
     serializer_name = "json"