3 files changed, 19 insertions, 2 deletions
diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index a022fe09..c406a2f3 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -186,8 +186,10 @@ class WritableField(Field):
             else:
                 native = data[field_name]
         except KeyError:
-            if self.default is not None:
+            if self.default is not None and not self.root.partial:
                 native = self.default
+                # partial serializers shouldn't set the default field to avoid
+                # overriding the previously set value
             else:
                 if self.required:
                     raise ValidationError(self.error_messages['required'])
diff --git a/rest_framework/tests/models.py b/rest_framework/tests/models.py
index 54fa2dec..59c35074 100644
--- a/rest_framework/tests/models.py
+++ b/rest_framework/tests/models.py
@@ -71,6 +71,7 @@ class SlugBasedModel(RESTFrameworkModel):
 
 class DefaultValueModel(RESTFrameworkModel):
     text = models.CharField(default='foobar', max_length=100)
+    extra = models.CharField(blank=True, null=True, max_length=100)
 
 
 class CallableDefaultValueModel(RESTFrameworkModel):
diff --git a/rest_framework/tests/serializer.py b/rest_framework/tests/serializer.py
index 701b2f47..8767385e 100644
--- a/rest_framework/tests/serializer.py
+++ b/rest_framework/tests/serializer.py
@@ -340,7 +340,6 @@ class ModelValidationTests(TestCase):
         self.assertTrue(photo_serializer.save())
 
 
-
 class RegexValidationTest(TestCase):
     def test_create_failed(self):
         serializer = BookSerializer(data={'isbn': '1234567890'})
@@ -551,6 +550,21 @@ class DefaultValueTests(TestCase):
         self.assertEquals(instance.pk, 1)
         self.assertEquals(instance.text, 'overridden')
 
+    def test_partial_update_default(self):
+        """ Regression test for issue #532 """
+        data = {'text': 'overridden'}
+        serializer = self.serializer_class(data=data, partial=True)
+        self.assertEquals(serializer.is_valid(), True)
+        instance = serializer.save()
+
+        data = {'extra': 'extra_value'}
+        serializer = self.serializer_class(instance=instance, data=data, partial=True)
+        self.assertEquals(serializer.is_valid(), True)
+        instance = serializer.save()
+
+        self.assertEquals(instance.extra, 'extra_value')
+        self.assertEquals(instance.text, 'overridden')
+
 
 class CallableDefaultValueTests(TestCase):
     def setUp(self):