diff --git a/sqlalchemy_utils/path.py b/sqlalchemy_utils/path.py
index 5780e2a..9538245 100644
--- a/sqlalchemy_utils/path.py
+++ b/sqlalchemy_utils/path.py
@@ -74,7 +74,7 @@ class AttrPath(object):
                     "Invert failed because property '%s' of class "
                     "%s has no backref." % (
                         prop.key,
-                        prop.mapper.class_.__name__
+                        prop.parent.class_.__name__
                     )
                 )
             return backref
@@ -90,7 +90,7 @@ class AttrPath(object):
             if result[0] is self.parts[0]:
                 class_ = self.class_
             else:
-                class_ = result[0].mapper.class_
+                class_ = result[0].parent.class_
             return self.__class__(
                 class_,
                 self.path[slice]
diff --git a/tests/test_path.py b/tests/test_path.py
index cdccff3..0dac1bc 100644
--- a/tests/test_path.py
+++ b/tests/test_path.py
@@ -46,6 +46,7 @@ class TestAttrPath(TestCase):
             self.Document.sections,
             self.Section.subsections
         ]
+        assert str(path.path) == 'sections.subsections'
 
     def test_len(self):
         len(AttrPath(self.SubSection, 'section.document')) == 2
@@ -77,6 +78,8 @@ class TestAttrPath(TestCase):
     def test_getitem_with_slice(self):
         path = AttrPath(self.SubSection, 'section.document')
         assert path[:] == AttrPath(self.SubSection, 'section.document')
+        assert path[:-1] == AttrPath(self.SubSection, 'section')
+        assert path[1:] == AttrPath(self.Section, 'document')
 
     def test_eq(self):
         assert (