Propagate nullability in ModelSerializer (#8116)

Propagate the nullability of underlying model fields in ModelSerializer
when those fields are marked as read only. This ensures the correct
generation of OpenAPI schemas.

Fix #8041.

rest_framework/serializers.py

@@ -1326,9 +1326,8 @@ class ModelSerializer(Serializer):
         """
         if extra_kwargs.get('read_only', False):
             for attr in [
-                'required', 'default', 'allow_blank', 'allow_null',
-                'min_length', 'max_length', 'min_value', 'max_value',
-                'validators', 'queryset'
+                'required', 'default', 'allow_blank', 'min_length',
+                'max_length', 'min_value', 'max_value', 'validators', 'queryset'
             ]:
                 kwargs.pop(attr, None)
 

tests/schemas/test_openapi.py

@@ -2,6 +2,7 @@ import uuid
 import warnings
 
 import pytest
+from django.db import models
 from django.test import RequestFactory, TestCase, override_settings
 from django.urls import path
 from django.utils.translation import gettext_lazy as _
@@ -110,6 +111,24 @@ class TestFieldMapping(TestCase):
         assert data['properties']['default_false']['default'] is False, "default must be false"
         assert 'default' not in data['properties']['without_default'], "default must not be defined"
 
+    def test_nullable_fields(self):
+        class Model(models.Model):
+            rw_field = models.CharField(null=True)
+            ro_field = models.CharField(null=True)
+
+        class Serializer(serializers.ModelSerializer):
+            class Meta:
+                model = Model
+                fields = ["rw_field", "ro_field"]
+                read_only_fields = ["ro_field"]
+
+        inspector = AutoSchema()
+
+        data = inspector.map_serializer(Serializer())
+        assert data['properties']['rw_field']['nullable'], "rw_field nullable must be true"
+        assert data['properties']['ro_field']['nullable'], "ro_field nullable must be true"
+        assert data['properties']['ro_field']['readOnly'], "ro_field read_only must be true"
+
 
 @pytest.mark.skipif(uritemplate is None, reason='uritemplate not installed.')
 class TestOperationIntrospection(TestCase):