Warn when extra_field_kwargs includes fields explicitly declared (related to #3460)

rest_framework/serializers.py

@@ -1062,12 +1062,26 @@ class ModelSerializer(Serializer):
         fields = OrderedDict()
 
         for field_name in field_names:
+            extra_field_kwargs = extra_kwargs.get(field_name, {})
+
             # If the field is explicitly declared on the class then use that.
             if field_name in declared_fields:
-                fields[field_name] = declared_fields[field_name]
+                assert not extra_field_kwargs, (
+                    "The field '{field_name}' was declared on serializer "
+                    "{serializer_class}, but has also been included in "
+                    "extra_fields_kwargs ({extra_field_kwargs}). "
+                    "This must be a mistake, because extra_field_kwargs are "
+                    "not applied to declared fields. For example, if you want "
+                    "the field to be read-only, you must explicitly pass "
+                    "read_only=True when declaring, instead of including the "
+                    "field in read_only_fields.".format(
+                        field_name=field_name,
+                        serializer_class=self.__class__.__name__,
+                        extra_field_kwargs=extra_field_kwargs
+                    )
+                )
                 continue
 
-            extra_field_kwargs = extra_kwargs.get(field_name, {})
             source = extra_field_kwargs.get('source', '*')
             if source == '*':
                 source = field_name

tests/test_model_serializer.py

@@ -1333,3 +1333,26 @@ class Issue6751Test(TestCase):
         serializer.save()
 
         self.assertEqual(instance.char_field, 'value changed by signal')
+
+class ExtraKwargsForDeclaredFieldsModel(models.Model):
+    test_field = models.CharField(max_length=64)
+
+class TestExtraKwargsForDeclaredFields(TestCase):
+    """
+    Passing extra_kwargs for a declared field should throw an AssertionError.
+    Particularly relevant for read_only_fields. See
+    https://github.com/encode/django-rest-framework/issues/3460.
+    """
+    def test_extra_kwargs_for_declared_fields(self):
+        class TestSerializer(serializers.ModelSerializer):
+            test_field = serializers.CharField()
+
+            class Meta:
+                model = ExtraKwargsForDeclaredFieldsModel
+                fields = ('test_field', )
+                read_only_fields = ('test_field', )
+
+        with pytest.raises(AssertionError) as e_info:
+            TestSerializer(data={'test_field': 'abc'}).fields
+
+        assert 'extra_field_kwargs' in str(e_info.value)