1) fix the 'new' Metadata caching to really work. :)

2) remove a useless comment in FieldMetadata

src/calibre/library/caches.py

@@ -509,6 +509,7 @@ class ResultCache(SearchQueryParser):
 
     def set(self, row, col, val, row_is_id=False):
         id = row if row_is_id else self._map_filtered[row]
+        self._data[id][self.FIELD_MAP['all_metadata']] = None
         self._data[id][col] = val
 
     def get(self, row, col, row_is_id=False):

src/calibre/library/database2.py

@@ -529,7 +529,7 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
         if mi is not None:
             return mi
 
-        mi = self.field_metadata.get_empty_metadata_instance()
+        mi = Metadata(None)
         self.data.set(idx, self.FIELD_MAP['all_metadata'], mi,
                       row_is_id = index_is_id)
 
@@ -539,7 +539,6 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
         for (author, author_sort) in aut_list:
             aum.append(author)
             aus[author] = author_sort
-        mi = self.field_metadata.get_empty_metadata_instance()
         mi.title       = self.title(idx, index_is_id=index_is_id)
         mi.authors     = aum
         mi.author_sort = self.author_sort(idx, index_is_id=index_is_id)

src/calibre/library/field_metadata.py

@@ -304,7 +304,6 @@ class FieldMetadata(dict):
 
     # search labels that are not db columns
     search_items = [    'all',
-#                        'date',
                         'search',
                     ]