* Removed SlugRelatedField optimization

* Moved compatibility check of to_fields to compat.py
* Expanded the check with a way that should work on django <1.6. Need to check trevis after pushing..

rest_framework/compat.py

@@ -138,6 +138,21 @@ def __init__(self, *args, **kwargs):
             super(MaxLengthValidator, self).__init__(*args, **kwargs)
 
 
+# ForeignKey's to_fields parameter was changed in Django 1.6 to an array named to_fields
+# to_fields is an array but django lets you only set one to_field for ForeignKeys
+# https://docs.djangoproject.com/en/1.7/ref/models/fields/#django.db.models.ForeignKey.to_field
+# see also ForeignKey and ForeignObject in
+# https://docs.djangoproject.com/en/1.7/_modules/django/db/models/fields/related/
+
+if django.VERSION >= (1, 6):
+    def get_to_field(field):
+        # This should work for casual ForeignKeys
+        return field.to_fields[0] if len(field.to_fields) else None
+else:
+    def get_to_field(field):
+        return field.to_field
+
+
 # URLValidator only accepts `message` in 1.6+
 if django.VERSION >= (1, 6):
     from django.core.validators import URLValidator

rest_framework/relations.py

@@ -303,14 +303,7 @@ def __init__(self, slug_field=None, **kwargs):
 
     def to_internal_value(self, data):
         try:
-            # Some tests do not have the full QuerySet API
-            # But we optimize SlugRelatedFields by only loading what we need
-            qs = self.get_queryset()
-            if hasattr(qs, 'only'):
-                return self.get_queryset().only(self.slug_field).get(**{self.slug_field: data})
-            else:
-                return self.get_queryset().get(**{self.slug_field: data})
-
+            return self.get_queryset().only(self.slug_field).get(**{self.slug_field: data})
         except ObjectDoesNotExist:
             self.fail('does_not_exist', slug_name=self.slug_field, value=smart_text(data))
         except (TypeError, ValueError):

rest_framework/utils/model_meta.py

@@ -9,6 +9,7 @@
 from django.core.exceptions import ImproperlyConfigured
 from django.db import models
 from django.utils import six
+from rest_framework.compat import get_to_field
 from rest_framework.compat import OrderedDict
 import inspect
 
@@ -97,18 +98,11 @@ def _get_forward_relationships(opts):
     """
     forward_relations = OrderedDict()
     for field in [field for field in opts.fields if field.serialize and field.rel]:
-        # For < django 1.6
-        if hasattr(field, 'to_fields'):
-            to_field = field.to_fields[0] if len(field.to_fields) else None
-        else:
-            to_field = None
-
         forward_relations[field.name] = RelationInfo(
             model_field=field,
             related=_resolve_model(field.rel.to),
             to_many=False,
-            # to_fields is an array but django lets you only set one to_field
-            to_field=to_field,
+            to_field=get_to_field(field),
             has_through_model=False
         )
 
@@ -135,17 +129,11 @@ def _get_reverse_relationships(opts):
     reverse_relations = OrderedDict()
     for relation in opts.get_all_related_objects():
         accessor_name = relation.get_accessor_name()
-        # For < django 1.6
-        if hasattr(relation.field, 'to_fields'):
-            to_field = relation.field.to_fields[0] if len(relation.field.to_fields) else None
-        else:
-            to_field = None
-
         reverse_relations[accessor_name] = RelationInfo(
             model_field=None,
             related=relation.model,
             to_many=relation.field.rel.multiple,
-            to_field=to_field,
+            to_field=get_to_field(relation.field),
             has_through_model=False
         )