adding_appendlist_field: adding tranformitive field

rest_framework/fields.py

@@ -1933,3 +1933,19 @@ class ModelField(Field):
         if is_protected_type(value):
             return value
         return self.model_field.value_to_string(obj)
+
+
+class TransformitiveFieldMixin():
+    """
+    it's an interface to allow any field to change the it's data
+    from the already database object if using the modelserializer
+    """
+
+    def apply_transformation(self, instance, data):
+        """
+        the transformation to be done by the inherited field to do it's work
+        :param instance:
+        :param data:
+        :return:
+        """
+        raise NotImplementedError("apply_tranformation is not implement for this field %s" % self.field_name)

rest_framework/serializers.py

@@ -29,7 +29,7 @@ from django.utils.translation import ugettext_lazy as _
 
 from rest_framework.compat import Mapping, postgres_fields, unicode_to_repr
 from rest_framework.exceptions import ErrorDetail, ValidationError
-from rest_framework.fields import get_error_detail, set_value
+from rest_framework.fields import get_error_detail, set_value, TransformitiveFieldMixin
 from rest_framework.settings import api_settings
 from rest_framework.utils import html, model_meta, representation
 from rest_framework.utils.field_mapping import (
@@ -901,6 +901,20 @@ class ModelSerializer(Serializer):
     # "HTTP 201 Created" responses.
     url_field_name = None
 
+    def to_internal_value(self, data):
+        """
+        adding functionality of changing data sent by the serializer
+        by the data inside the instance
+        :param data:
+        :return:
+        """
+        fields = self._writable_fields
+        for field in fields:
+            if self.instance and isinstance(field, TransformitiveFieldMixin):
+                field.apply_transformation(self.instance, data)
+
+        return super(ModelSerializer, self).to_internal_value(data)
+
     # Default `create` and `update` behavior...
     def create(self, validated_data):
         """

tests/test_model_serializer.py

@@ -1274,3 +1274,50 @@ class Issue6110Test(TestCase):
         msginitial = ('Got a `TypeError` when calling `Issue6110TestModel.all_objects.create()`.')
         with self.assertRaisesMessage(TypeError, msginitial):
             Issue6110ModelSerializer().create({'wrong_param': 'wrong_param'})
+
+
+class TestTransformitiveField(TestCase):
+
+    def setUp(self):
+        self.instance = OneFieldModel(char_field="abc")
+
+    def __create_serializer(self,implementor):
+
+        class serializer(serializers.ModelSerializer):
+            char_field = implementor()
+            class Meta:
+                model = OneFieldModel
+                fields = ('char_field')
+        return serializer
+
+    def test_adding_transformitive_field_mixin_without_implementing_the_method(self):
+
+        class EmptyImplementor(serializers.TransformitiveFieldMixin,serializers.CharField):
+            pass
+
+        with self.assertRaises(NotImplementedError):
+            TestSerializer = self.__create_serializer(EmptyImplementor)
+            serializer = TestSerializer(instance=self.instance, data={})
+            serializer.save()
+
+    def test_adding_transformitive_field_mixin_with_implementing_the_method(self):
+
+        class NonEmptyImplementor(serializers.TransformitiveFieldMixin,serializers.CharField):
+            def apply_transformation(self, instance, data):
+                data["char_field"] = data["char_field"][0:1]+instance.char_field[0:1]
+
+        TestSerializer = self.__create_serializer(NonEmptyImplementor)
+        serializer = TestSerializer(instance=self.instance, data={"char_field":"bb"})
+        new_instance = serializer.save()
+        self.asserEqual(new_instance.char_field,"ba")
+
+    def test_if_no_instance_is_passed_no_errors_would_happen(self):
+
+        class NonEmptyImplementor(serializers.TransformitiveFieldMixin,serializers.CharField):
+            def apply_transformation(self, instance, data):
+                data["char_field"] = data["char_field"][0:1]+instance.char_field[0:1]
+
+        TestSerializer = self.__create_serializer(NonEmptyImplementor)
+        data={"char_field":"bb"}
+        serializer = TestSerializer()
+        serializer.to_internal_value(data)