Respect to_field property of ForeignKey relations

rest_framework/compat.py

@@ -161,6 +161,21 @@ 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.rel.field_name
+
+
 # URLValidator only accepts `message` in 1.6+
 if django.VERSION >= (1, 6):
     from django.core.validators import URLValidator

rest_framework/serializers.py

@@ -732,6 +732,7 @@ class ModelSerializer(Serializer):
         models.URLField: URLField,
     }
     serializer_related_field = PrimaryKeyRelatedField
+    serializer_related_to_field = SlugRelatedField
     serializer_url_field = HyperlinkedIdentityField
     serializer_choice_field = ChoiceField
 
@@ -1021,6 +1022,11 @@ def build_relational_field(self, field_name, relation_info):
         field_class = self.serializer_related_field
         field_kwargs = get_relation_kwargs(field_name, relation_info)
 
+        to_field = field_kwargs.pop('to_field', None)
+        if to_field and to_field != 'id':
+            field_kwargs['slug_field'] = to_field
+            field_class = self.serializer_related_to_field
+
         # `view_name` is only valid for hyperlinked relationships.
         if not issubclass(field_class, HyperlinkedRelatedField):
             field_kwargs.pop('view_name', None)

rest_framework/utils/field_mapping.py

@@ -197,7 +197,7 @@ 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)
@@ -206,6 +206,9 @@ 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_model',
     'to_many',
+    'to_field',
     'has_through_model'
 ])
 
@@ -100,6 +102,7 @@ def _get_forward_relationships(opts):
             model_field=field,
             related_model=_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_model=_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
             )
@@ -133,6 +138,7 @@ def _get_reverse_relationships(opts):
             model_field=None,
             related_model=related,
             to_many=relation.field.rel.multiple,
+            to_field=get_to_field(relation.field),
             has_through_model=False
         )
 
@@ -144,6 +150,8 @@ def _get_reverse_relationships(opts):
             model_field=None,
             related_model=related,
             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