Add SerializerMethodField

docs/api-guide/fields.md

@@ -324,5 +324,11 @@ This field is always read-only.
 * `pk_url_kwarg` - The named url parameter for the pk field lookup. Default is `pk`.
 * `slug_url_kwarg` - The named url parameter for the slug field lookup. Default is to use the same value as given for `slug_field`.
 
+# Other Fields
+
+## SerializerMethodField
+
+This is a read-only field gets its value by calling a method on the serializer class it's attached to. It can be used to add any sort of data to the serialized representation of your object. The field's constructor accepts a single argument, which is the name of the method on the serializer to be called. The method should accept a single argument (in addition to `self`), which is the object being serialized. It should return whatever you want to be included in the serialized representation of the object.
+
 [cite]: http://www.python.org/dev/peps/pep-0020/
 [FILE_UPLOAD_HANDLERS]: https://docs.djangoproject.com/en/dev/ref/settings/#std:setting-FILE_UPLOAD_HANDLERS

rest_framework/fields.py

@@ -1019,3 +1019,17 @@ class ImageField(FileField):
         if hasattr(f, 'seek') and callable(f.seek):
             f.seek(0)
         return f
+
+
+class SerializerMethodField(Field):
+    """
+    A field that gets its value by calling a method on the serializer it's attached to.
+    """
+
+    def __init__(self, method_name):
+        self.method_name = method_name
+        super(SerializerMethodField, self).__init__()
+
+    def field_to_native(self, obj, field_name):
+        value = getattr(self.parent, self.method_name)(obj)
+        return self.to_native(value)

rest_framework/tests/serializer.py

@@ -497,6 +497,40 @@ class ManyRelatedTests(TestCase):
         self.assertEqual(serializer.data, expected)
 
 
+class SerializerMethodFieldTests(TestCase):
+    def setUp(self):
+
+        class BoopSerializer(serializers.Serializer):
+            beep = serializers.SerializerMethodField('get_beep')
+            boop = serializers.Field()
+            boop_count = serializers.SerializerMethodField('get_boop_count')
+
+            def get_beep(self, obj):
+                return 'hello!'
+
+            def get_boop_count(self, obj):
+                return len(obj.boop)
+
+        self.serializer_class = BoopSerializer
+
+    def test_serializer_method_field(self):
+
+        class MyModel(object):
+            boop = ['a', 'b', 'c']
+
+        source_data = MyModel()
+
+        serializer = self.serializer_class(source_data)
+
+        expected = {
+            'beep': u'hello!',
+            'boop': [u'a', u'b', u'c'],
+            'boop_count': 3,
+        }
+
+        self.assertEqual(serializer.data, expected)
+
+
 # Test for issue #324
 class BlankFieldTests(TestCase):
     def setUp(self):