From 3f50c415c403383f088ecb49385e5f9586955ebd Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Sat, 3 Sep 2011 08:52:30 -0600
Subject: [PATCH] ...

---
 src/calibre/db/cache.py  |  5 +++--
 src/calibre/db/fields.py | 14 ++++++--------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/src/calibre/db/cache.py b/src/calibre/db/cache.py
index 7158fc0267..01e97bb4c3 100644
--- a/src/calibre/db/cache.py
+++ b/src/calibre/db/cache.py
@@ -347,8 +347,9 @@ class Cache(object):
                     as_path=as_path)
 
     @read_api
-    def multisort(self, fields):
-        all_book_ids = frozenset(self._all_book_ids())
+    def multisort(self, fields, ids_to_sort=None):
+        all_book_ids = frozenset(self._all_book_ids() if ids_to_sort is None
+                else ids_to_sort)
         get_metadata = partial(self._get_metadata, get_user_categories=False)
 
         sort_keys = tuple(self.fields[field[0]].sort_keys_for_books(get_metadata,
diff --git a/src/calibre/db/fields.py b/src/calibre/db/fields.py
index 7fd19501ce..0a497555c4 100644
--- a/src/calibre/db/fields.py
+++ b/src/calibre/db/fields.py
@@ -51,9 +51,13 @@ class Field(object):
 
     def __iter__(self):
         '''
-        Iterate over the ids for all values in this field
+        Iterate over the ids for all values in this field.
+
+        WARNING: Some fields such as composite fields and virtual
+        fields like ondevice do not have ids for their values, in such
+        cases this is an empty iterator.
         '''
-        raise NotImplementedError()
+        return iter(())
 
     def sort_keys_for_books(self, get_metadata, all_book_ids):
         '''
@@ -78,9 +82,6 @@ class OneToOneField(Field):
     def __iter__(self):
         return self.table.book_col_map.iterkeys()
 
-    def iter_book_ids(self):
-        return self.table.book_col_map.iterkeys()
-
     def sort_keys_for_books(self, get_metadata, all_book_ids):
         return {id_ : self._sort_key(self.book_col_map.get(id_, '')) for id_ in
                 all_book_ids}
@@ -154,9 +155,6 @@ class OnDeviceField(OneToOneField):
     def __iter__(self):
         return iter(())
 
-    def iter_book_ids(self):
-        return iter(())
-
     def sort_keys_for_books(self, get_metadata, all_book_ids):
         return {id_ : self.for_book(id_) for id_ in
                 all_book_ids}