destination now uses album values when available

When computing track destination paths, we now look for album-level values when
they're available. This has the effect of making albums go into a single
directory even when their tracks have heterogeneous metadata. We will need to
revisit this once we start explicitly supporting non-album tracks.

beets/library.py

@@ -805,13 +805,23 @@ class Library(BaseLibrary):
         item (i.e., where the file ought to be).
         """
         subpath_tmpl = Template(self.path_format)
+
+        # Get the item's Album if it has one.
+        album = self.get_album(item)
         
-        # build the mapping for substitution in the path template, beginning
-        # with the values from the database
+        # Build the mapping for substitution in the path template,
+        # beginning with the values from the database.
         mapping = {}
         for key in ITEM_KEYS_META:
-            value = getattr(item, key)
-            # sanitize the value for inclusion in a path:
+            # Get the values from either the item or its album.
+            if key in ALBUM_KEYS_ITEM and album is not None:
+                # From album.
+                value = getattr(album, key)
+            else:
+                # From Item.
+                value = getattr(item, key)
+
+            # Sanitize the value for inclusion in a path:
             # replace / and leading . with _
             if isinstance(value, basestring):
                 value = value.replace(os.sep, '_')

test/test_db.py

@@ -254,6 +254,14 @@ class DestinationTest(unittest.TestCase):
         p = self.lib.destination(self.i)
         self.assert_('(FLAC)' in p)
 
+    def test_heterogeneous_album_gets_single_directory(self):
+        i1, i2 = item(), item()
+        self.lib.add_album([i1, i2])
+        i1.year, i2.year = 2009, 2010
+        self.lib.path_format = '$album ($year)/$track $title'
+        dest1, dest2 = self.lib.destination(i1), self.lib.destination(i2)
+        self.assertEqual(os.path.dirname(dest1), os.path.dirname(dest2))
+
 class MigrationTest(unittest.TestCase):
     """Tests the ability to change the database schema between
     versions.