Set up serializer fields lazily on-demand.

This avoids AppRegistryNotReady problems in Django 1.7 with nested serializers,
which are instantiated at import time, possibly before Django's app registry is
fully populated.

rest_framework/serializers.py

@@ -22,6 +22,7 @@ from django.db import models
 from django.forms import widgets
 from django.utils import six
 from django.utils.datastructures import SortedDict
+from django.utils.functional import cached_property
 from django.core.exceptions import ObjectDoesNotExist
 from rest_framework.settings import api_settings
 
@@ -197,7 +198,6 @@ class BaseSerializer(WritableField):
         self.init_data = data
         self.init_files = files
         self.object = instance
-        self.fields = self.get_fields()
 
         self._data = None
         self._files = None
@@ -212,6 +212,10 @@ class BaseSerializer(WritableField):
     #####
     # Methods to determine which fields to use when (de)serializing objects.
 
+    @cached_property
+    def fields(self):
+        return self.get_fields()
+
     def get_default_fields(self):
         """
         Return the complete set of default fields for the object, as a dict.

tests/test_relations.py

@@ -102,7 +102,7 @@ class RelatedFieldSourceTests(TestCase):
         self.assertEqual(value, ['BlogPost object'])
 
     # Regression for #1129
-    def test_exception_for_incorect_fk(self):
+    def test_exception_for_incorrect_fk(self):
         """
         Check that the exception message are correct if the source field
         doesn't exist.
@@ -123,8 +123,9 @@ class RelatedFieldSourceTests(TestCase):
             (serializers.ModelSerializer,),
             attrs
         )
+        serializer = TestSerializer(data={'name': 'foo'})
         with self.assertRaises(AttributeError):
-            TestSerializer(data={'name': 'foo'})
+            serializer.fields
 
 
 @unittest.skipIf(get_version() < '1.6.0', 'Upstream behaviour changed in v1.6')

tests/test_serializer.py

@@ -327,7 +327,9 @@ class BasicTests(TestCase):
         """
         Regression test for #652.
         """
-        self.assertRaises(AssertionError, PersonSerializerInvalidReadOnly, [])
+        serializer = PersonSerializerInvalidReadOnly()
+        with self.assertRaises(AssertionError):
+            serializer.fields
 
     def test_serializer_data_is_cleared_on_save(self):
         """