Clear cached serializer data on `save()` + test. Fixes #1116.

rest_framework/serializers.py

@@ -518,6 +518,9 @@ class BaseSerializer(WritableField):
         """
         Save the deserialized object and return it.
         """
+        # Clear cached _data, which may be invalidated by `save()`
+        self._data = None
+
         if isinstance(self.object, list):
             [self.save_object(item, **kwargs) for item in self.object]
 

rest_framework/tests/test_serializer.py

+# -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 from django.db import models
 from django.db.models.fields import BLANK_CHOICE_DASH
@@ -136,6 +137,7 @@ class BasicTests(TestCase):
             'Happy new year!',
             datetime.datetime(2012, 1, 1)
         )
+        self.actionitem = ActionItem(title='Some to do item',)
         self.data = {
             'email': 'tom@example.com',
             'content': 'Happy new year!',
@@ -264,6 +266,20 @@ class BasicTests(TestCase):
         """
         self.assertRaises(AssertionError, PersonSerializerInvalidReadOnly, [])
 
+    def test_serializer_data_is_cleared_on_save(self):
+        """
+        Check _data attribute is cleared on `save()`
+
+        Regression test for #1116
+            — id field is not populated is `data` is accessed prior to `save()`
+        """
+        serializer = ActionItemSerializer(self.actionitem)
+        self.assertIsNone(serializer.data.get('id',None), 'New instance. `id` should not be set.')
+        serializer.save()
+        self.assertIsNotNone(serializer.data.get('id',None), 'Model is saved. `id` should be set.')
+
+
+
 
 class DictStyleSerializer(serializers.Serializer):
     """