Fix 'count-of' searches (e.g., tags:#>3). Add a small blurb to the manual.

src/calibre/library/caches.py

@@ -406,11 +406,9 @@ class ResultCache(SearchQueryParser): # {{{
         if val_func is None:
             loc = self.field_metadata[location]['rec_index']
             val_func = lambda item, loc=loc: item[loc]
-        dt = self.field_metadata[location]['datatype']
-
         q = ''
-        val_func = lambda item, loc=loc: item[loc]
         cast = adjust = lambda x: x
+        dt = self.field_metadata[location]['datatype']
 
         if query == 'false':
             if dt == 'rating' or location == 'cover':

src/calibre/manual/gui.rst

@@ -365,6 +365,8 @@ Dates and numeric fields support the relational operators ``=`` (equals), ``>``
 Rating fields are considered to be numeric. For example, the search ``rating:>=3`` will find all books rated 3
 or higher.
 
+You can search for the number of items in multiple-valued fields such as tags). These searches begin with the character ``#``, then use the same syntax as numeric fields. For example, to find all books with more than 4 tags, use ``tags:#>4``. To find all books with exactly 10 tags, use ``tags:#=10``.
+
 Series indices are searchable. For the standard series, the search name is 'series_index'. For
 custom series columns, use the column search name followed by _index. For example, to search the indices for a
 custom series column named ``#my_series``, you would use the search name ``#my_series_index``.