Merge pull request #1071 from craigds/field-transform-methods

Feature: add transform_<fieldname> methods to serializers

docs/api-guide/serializers.md

@@ -67,6 +67,21 @@ At this point we've translated the model instance into Python native datatypes.
     json
     # '{"email": "leila@example.com", "content": "foo bar", "created": "2012-08-22T16:20:09.822"}'
 
+### Customizing field representation
+
+Sometimes when serializing objects, you may not want to represent everything exactly the way it is in your model.
+
+If you need to customize the serialized value of a particular field, you can do this by creating a `transform_<fieldname>` method. For example if you needed to render some markdown from a text field:
+
+    description = serializers.TextField()
+    description_html = serializers.TextField(source='description', read_only=True)
+
+    def transform_description_html(self, obj, value):
+        from django.contrib.markup.templatetags.markup import markdown
+        return markdown(value)
+
+These methods are essentially the reverse of `validate_<fieldname>` (see *Validation* below.)
+
 ## Deserializing objects
         
 Deserialization is similar.  First we parse a stream into Python native datatypes... 

rest_framework/serializers.py

@@ -307,6 +307,9 @@ class BaseSerializer(WritableField):
             field.initialize(parent=self, field_name=field_name)
             key = self.get_field_key(field_name)
             value = field.field_to_native(obj, field_name)
+            method = getattr(self, 'transform_%s' % field_name, None)
+            if callable(method):
+                value = method(obj, value)
             ret[key] = value
             ret.fields[key] = field
         return ret

rest_framework/tests/test_serializer.py

@@ -1659,3 +1659,38 @@ class SerializerSupportsManyRelationships(TestCase):
         serializer = SimpleSlugSourceModelSerializer(data={'text': 'foo', 'targets': [1, 2]})
         self.assertTrue(serializer.is_valid())
         self.assertEqual(serializer.data, {'text': 'foo', 'targets': [1, 2]})
+
+
+class TransformMethodsSerializer(serializers.Serializer):
+    a = serializers.CharField()
+    b_renamed = serializers.CharField(source='b')
+
+    def transform_a(self, obj, value):
+        return value.lower()
+
+    def transform_b_renamed(self, obj, value):
+        if value is not None:
+            return 'and ' + value
+
+
+class TestSerializerTransformMethods(TestCase):
+    def setUp(self):
+        self.s = TransformMethodsSerializer()
+
+    def test_transform_methods(self):
+        self.assertEqual(
+            self.s.to_native({'a': 'GREEN EGGS', 'b': 'HAM'}),
+            {
+                'a': 'green eggs',
+                'b_renamed': 'and HAM',
+            }
+        )
+
+    def test_missing_fields(self):
+        self.assertEqual(
+            self.s.to_native({'a': 'GREEN EGGS'}),
+            {
+                'a': 'green eggs',
+                'b_renamed': None,
+            }
+        )