Respect to_field property of ForeignKey relations

rest_framework/compat.py

@@ -146,6 +146,21 @@ 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

@@ -303,7 +303,7 @@ def __init__(self, slug_field=None, **kwargs):
 
     def to_internal_value(self, data):
         try:
-            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/serializers.py

@@ -727,6 +727,7 @@ class ModelSerializer(Serializer):
         models.URLField: URLField,
     })
     _related_class = PrimaryKeyRelatedField
+    _related_to_field_class = SlugRelatedField
 
     def create(self, validated_data):
         """
@@ -1002,8 +1003,16 @@ def get_fields(self):
                     field_cls = self._get_nested_class(depth, relation_info)
                     kwargs = get_nested_relation_kwargs(relation_info)
                 else:
-                    field_cls = self._related_class
                     kwargs = get_relation_kwargs(field_name, relation_info)
+                    to_field = kwargs.pop('to_field', None)
+                    # it seems that some tests/django initializers are setting
+                    # to_field where it is totally unnecessary
+                    if to_field and to_field != 'id':
+                        # using the slug field for now
+                        kwargs['slug_field'] = to_field
+                        field_cls = self._related_to_field_class
+                    else:
+                        field_cls = self._related_class
                     # `view_name` is only valid for hyperlinked relationships.
                     if not issubclass(field_cls, HyperlinkedRelatedField):
                         kwargs.pop('view_name', None)

rest_framework/utils/field_mapping.py

@@ -194,7 +194,7 @@ def get_relation_kwargs(field_name, relation_info):
     """
     Creates a default instance of a flat relational field.
     """
-    model_field, related_model, to_many, has_through_model = relation_info
+    model_field, related_model, to_many, to_field, has_through_model = relation_info
     kwargs = {
         'queryset': related_model._default_manager,
         'view_name': get_detail_view_name(related_model)
@@ -203,6 +203,9 @@ def get_relation_kwargs(field_name, relation_info):
     if to_many:
         kwargs['many'] = True
 
+    if to_field:
+        kwargs['to_field'] = to_field
+
     if has_through_model:
         kwargs['read_only'] = True
         kwargs.pop('queryset', None)

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
 
@@ -26,6 +27,7 @@
     'model_field',
     'related',
     'to_many',
+    'to_field',
     'has_through_model'
 ])
 
@@ -100,6 +102,7 @@ def _get_forward_relationships(opts):
             model_field=field,
             related=_resolve_model(field.rel.to),
             to_many=False,
+            to_field=get_to_field(field),
             has_through_model=False
         )
 
@@ -109,6 +112,8 @@ def _get_forward_relationships(opts):
             model_field=field,
             related=_resolve_model(field.rel.to),
             to_many=True,
+            # manytomany do not have to_fields
+            to_field=None,
             has_through_model=(
                 not field.rel.through._meta.auto_created
             )
@@ -128,6 +133,7 @@ def _get_reverse_relationships(opts):
             model_field=None,
             related=relation.model,
             to_many=relation.field.rel.multiple,
+            to_field=get_to_field(relation.field),
             has_through_model=False
         )
 
@@ -138,6 +144,8 @@ def _get_reverse_relationships(opts):
             model_field=None,
             related=relation.model,
             to_many=True,
+            # manytomany do not have to_fields
+            to_field=None,
             has_through_model=(
                 (getattr(relation.field.rel, 'through', None) is not None)
                 and not relation.field.rel.through._meta.auto_created