Fixes sorting by multiple fields on a relation

CHANGES

@@ -28,6 +28,8 @@ Not yet released.
 - #431: adds a ``url_prefix`` keyword argument to the :class:`APIManager`
   constructor, so one can specify a URL prefix once for all created APIs.
 - #449: roll back the session on any SQLAlchemy error, not just a few.
+- #432, #462: alias relation names when sorting by multiple attributes on a
+  relationship.
 - #436, #453: use ``__table__.name`` instead of ``__tablename__`` to infer the
   collection name for the SQLAlchemy model.
 - #440, #475: uses the serialization function provided at the time of invoking

flask_restless/search.py

@@ -22,6 +22,7 @@
 from sqlalchemy import and_
 from sqlalchemy import or_
 from sqlalchemy.ext.associationproxy import AssociationProxy
+from sqlalchemy.orm import aliased
 from sqlalchemy.orm.attributes import InstrumentedAttribute
 
 from .helpers import get_model
@@ -414,7 +415,7 @@ def search(session, model, filters=None, sort=None, group_by=None,
             direction_name = 'asc' if symbol == '+' else 'desc'
             if '.' in field_name:
                 field_name, field_name_in_relation = field_name.split('.')
-                relation_model = get_related_model(model, field_name)
+                relation_model = aliased(get_related_model(model, field_name))
                 field = getattr(relation_model, field_name_in_relation)
                 direction = getattr(field, direction_name)
                 query = query.join(relation_model)

tests/test_jsonapi/test_fetching_data.py

@@ -910,6 +910,32 @@ def test_sort_relationship_attributes(self):
         articles = document['data']
         assert ['2', '1', '3'] == [c['id'] for c in articles]
 
+
+    def test_sort_multiple_relationship_attributes(self):
+        """Tests that the client can sort by multiple relationship
+        attributes.
+
+        For more information, see the `Sorting`_ section of the JSON API
+        specification.
+
+        .. _Sorting: http://jsonapi.org/format/#fetching-sorting
+
+        """
+        person1 = self.Person(age=2, name=u'd')
+        person2 = self.Person(age=1, name=u'b')
+        person3 = self.Person(age=1, name=u'a')
+        person4 = self.Person(age=2, name=u'c')
+        people = [person1, person2, person3, person4]
+        articles = [self.Article(id=i, author=person)
+                    for i, person in enumerate(people, start=1)]
+        self.session.add_all(people + articles)
+        self.session.commit()
+        query_string = {'sort': 'author.age,author.name'}
+        response = self.app.get('/api/article', query_string=query_string)
+        document = loads(response.data)
+        articles = document['data']
+        assert ['3', '2', '4', '1'] == [c['id'] for c in articles]
+
     def test_sorting_relationship(self):
         """Tests for sorting relationship objects when requesting
         information from a to-many relationship endpoint.