Avoid overeager inclusion of album attributes

Author: ctrueden

beets/dbcore/db.py

@@ -56,10 +56,11 @@ class FormattedMapping(Mapping):
     are replaced.
     """
 
-    def __init__(self, model, for_path=False):
+    def __init__(self, model, for_path=False, compute_keys=True):
         self.for_path = for_path
         self.model = model
-        self.model_keys = model.keys(True)
+        if compute_keys:
+            self.model_keys = model.keys(True)
 
     def __getitem__(self, key):
         if key in self.model_keys:

beets/library.py

@@ -375,7 +375,7 @@ class FormattedItemMapping(dbcore.db.FormattedMapping):
     """
 
     def __init__(self, item, for_path=False):
-        super(FormattedItemMapping, self).__init__(item, for_path)
+        super(FormattedItemMapping, self).__init__(item, for_path, compute_keys=False)
         # We treat album and item keys specially here,
         # so exclude transitive album keys from the model's keys.
         self.model_keys = item.keys(computed=True, with_album=False)