Guard against the possible misspelling `readonly_fields` in model serializers

Fixes #4897.

rest_framework/serializers.py

@@ -1290,6 +1290,15 @@ class ModelSerializer(Serializer):
                 kwargs['read_only'] = True
                 extra_kwargs[field_name] = kwargs
 
+        else:
+            # Guard against the possible misspelling `readonly_fields` (used
+            # by the Django admin and others).
+            assert not hasattr(self.Meta, 'readonly_fields'), (
+                'Serializer `%s.%s` has field `readonly_fields`; '
+                'the correct spelling for the option is `read_only_fields`.' %
+                (self.__class__.__module__, self.__class__.__name__)
+            )
+
         return extra_kwargs
 
     def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs):

tests/test_model_serializer.py

@@ -10,6 +10,7 @@ from __future__ import unicode_literals
 import decimal
 from collections import OrderedDict
 
+import pytest
 from django.core.exceptions import ImproperlyConfigured
 from django.core.validators import (
     MaxValueValidator, MinLengthValidator, MinValueValidator
@@ -1064,3 +1065,18 @@ class Issue3674Test(TestCase):
 
         child_expected = {'parent': 1, 'value': 'def'}
         self.assertEqual(child_serializer.data, child_expected)
+
+
+class Issue4897TestCase(TestCase):
+    def test_should_assert_if_writing_readonly_fields(self):
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = OneFieldModel
+                fields = ('char_field',)
+                readonly_fields = fields
+
+        obj = OneFieldModel.objects.create(char_field='abc')
+
+        with pytest.raises(AssertionError) as cm:
+            TestSerializer(obj).fields
+        cm.match(r'readonly_fields')