...

src/calibre/db/cache.py

@@ -120,10 +120,9 @@ class Cache(object):
         mi.ondevice_col = self._field_for('ondevice', book_id, default_value='')
         mi.last_modified = self._field_for('last_modified', book_id,
                 default_value=n)
-        formats = self._field_for('formats', book_id, default_value=())
+        formats = self._field_for('formats', book_id)
         mi.format_metadata = {}
-        mi.languages = list(self._field_for('languages', book_id,
-            default_value=()))
+        mi.languages = list(self._field_for('languages', book_id))
         if not formats:
             good_formats = None
         else:
@@ -208,16 +207,13 @@ class Cache(object):
         ``book_id``. If no such book exists or it has no defined value for the
         field ``name`` or no such field exists, then ``default_value`` is returned.
 
-        default_values is not used for title, title_sort, authors, author_sort
+        default_value is not used for title, title_sort, authors, author_sort
         and series_index. This is because these always have values in the db.
         default_value is used for all custom columns.
 
-        The returned value for is_multiple fields are always tuples, unless
-        default_value is returned.
-
-        WARNING: When returning the value for a is_multiple custom field this
-        method returns None (the default_value) if no value is set. The
-        get_custom() method from the old interface returned []
+        The returned value for is_multiple fields are always tuples, even when
+        no values are found (in other words, default_value is ignored). The
+        exception is identifiers for which the returned value is always a dict.
 
         WARNING: For is_multiple fields this method returns tuples, the old
         interface generally returned lists.
@@ -230,7 +226,13 @@ class Cache(object):
             return self.composite_for(name, book_id,
                     default_value=default_value)
         try:
-            return self.fields[name].for_book(book_id, default_value=default_value)
+            field = self.fields[name]
+        except KeyError:
+            return default_value
+        if field.is_multiple:
+            default_value = {} if name == 'identifiers' else ()
+        try:
+            return field.for_book(book_id, default_value=default_value)
         except (KeyError, IndexError):
             return default_value
 

src/calibre/db/fields.py

@@ -33,6 +33,8 @@ class Field(object):
             self._default_sort_key = UNDEFINED_DATE
         if self.name == 'languages':
             self._sort_key = lambda x:sort_key(calibre_langcode_to_name(x))
+        self.is_multiple = (bool(self.metadata['is_multiple']) or self.name ==
+                'formats')
 
     @property
     def metadata(self):
@@ -141,6 +143,7 @@ class OnDeviceField(OneToOneField):
     def __init__(self, name, table):
         self.name = name
         self.book_on_device_func = None
+        self.is_multiple = False
 
     def book_on_device(self, book_id):
         if callable(self.book_on_device_func):

src/calibre/db/tests/reading.py

@@ -34,9 +34,9 @@ class ReadingTest(BaseTest):
                     'series' : None,
                     'series_index': 1.0,
                     'rating': None,
-                    'tags': None,
-                    'formats':None,
-                    'identifiers': None,
+                    'tags': (),
+                    'formats':(),
+                    'identifiers': {},
                     'timestamp': datetime.datetime(2011, 9, 7, 13, 54, 41,
                         tzinfo=local_tz),
                     'pubdate': datetime.datetime(2011, 9, 7, 13, 54, 41,
@@ -44,15 +44,15 @@ class ReadingTest(BaseTest):
                     'last_modified': datetime.datetime(2011, 9, 7, 13, 54, 41,
                         tzinfo=local_tz),
                     'publisher': None,
-                    'languages': None,
+                    'languages': (),
                     'comments': None,
                     '#enum': None,
-                    '#authors':None,
+                    '#authors':(),
                     '#date':None,
                     '#rating':None,
                     '#series':None,
                     '#series_index': None,
-                    '#tags':None,
+                    '#tags':(),
                     '#yesno':None,
                     '#comments': None,
 
@@ -66,7 +66,7 @@ class ReadingTest(BaseTest):
                     'series' : 'Series One',
                     'series_index': 1.0,
                     'tags':('Tag Two', 'Tag One'),
-                    'formats': None,
+                    'formats': (),
                     'rating': 4.0,
                     'identifiers': {'test':'one'},
                     'timestamp': datetime.datetime(2011, 9, 5, 15, 6,
@@ -96,7 +96,7 @@ class ReadingTest(BaseTest):
                     'series_index': 2.0,
                     'rating': 6.0,
                     'tags': ('Tag One',),
-                    'formats':None,
+                    'formats':(),
                     'identifiers': {'test':'two'},
                     'timestamp': datetime.datetime(2011, 9, 6, 0, 0,
                         tzinfo=local_tz),
@@ -120,8 +120,10 @@ class ReadingTest(BaseTest):
         }
         for book_id, test in tests.iteritems():
             for field, expected_val in test.iteritems():
-                self.assertEqual(expected_val,
-                        cache.field_for(field, book_id))
+                val = cache.field_for(field, book_id)
+                self.assertEqual(expected_val, val,
+                        'Book id: %d Field: %s failed: %r != %r'%(
+                            book_id, field, expected_val, val))
         # }}}
 
     def test_sorting(self): # {{{