Allow setting read_only fields from defaults

According to the documentation, this is supposed to work, but it doesn't. Add the read_only default values after the validation runs so they actually get saved.

rest_framework/serializers.py

@@ -488,6 +488,7 @@ class Serializer(BaseSerializer, metaclass=SerializerMetaclass):
         if errors:
             raise ValidationError(errors)
 
+        ret.update(self._read_only_defaults())
         return ret
 
     def to_representation(self, instance):

tests/test_model_serializer.py

@@ -1333,3 +1333,18 @@ class Issue6751Test(TestCase):
         serializer.save()
 
         self.assertEqual(instance.char_field, 'value changed by signal')
+
+
+class TestReadonlyDefault(TestCase):
+    def test_readonly_default(self):
+        class TestSerializer(serializers.ModelSerializer):
+            char_field = serializers.CharField(read_only=True, default='default')
+
+            class Meta:
+                model = OneFieldModel
+                fields = ('char_field',)
+
+        serializer = TestSerializer(data={})
+        serializer.is_valid()
+        instance = serializer.save()
+        assert instance.char_field == 'default'

tests/test_serializer_nested.py

@@ -306,6 +306,35 @@ class TestNestedWriteErrors(TestCase):
         )
 
 
+class TestNestedWriteReadonlyDefault(TestCase):
+    def test_nested_serializer_readonly_default(self):
+        profile_obj = NestedWriteProfile.objects.create(address='52 festive road')
+
+        class ProfileSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = NestedWriteProfile
+                fields = ['address']
+
+        class NestedProfileSerializer(serializers.ModelSerializer):
+            profile = ProfileSerializer(read_only=True, default=lambda: profile_obj)
+
+            class Meta:
+                model = NestedWritePerson
+                fields = ['profile']
+
+        serializer = NestedProfileSerializer(data={})
+        assert serializer.is_valid()
+        assert serializer.validated_data == {'profile': profile_obj}
+        nested = serializer.save()
+        assert nested.profile == profile_obj
+
+        serializer = NestedProfileSerializer(data={'profile': {'address': '123 fake street'}})
+        assert serializer.is_valid()
+        assert serializer.validated_data == {'profile': profile_obj}
+        serializer.save()
+        assert nested.profile == profile_obj
+
+
 if postgres_fields:
     class NonRelationalPersonModel(models.Model):
         """Model declaring a postgres JSONField"""