First stage: adding a ProxyMetadata object to get_metadata

src/calibre/db/cache.py

@@ -184,6 +184,9 @@ class Cache(object):
 
     def _get_metadata(self, book_id, get_user_categories=True):  # {{{
         mi = Metadata(None, template_cache=self.formatter_template_cache)
+
+        mi._proxy_metadata = ProxyMetadata(self, book_id, formatter=mi.formatter)
+
         author_ids = self._field_ids_for('authors', book_id)
         adata = self._author_data(author_ids)
         aut_list = [adata[i] for i in author_ids]

src/calibre/db/lazy.py

@@ -278,8 +278,9 @@ for field in ('formats', 'format_metadata'):
 
 class ProxyMetadata(Metadata):
 
-    def __init__(self, db, book_id):
+    def __init__(self, db, book_id, formatter=None):
         sa(self, 'template_cache', db.formatter_template_cache)
+        if formatter is None:
             sa(self, 'formatter', SafeFormat())
         sa(self, '_db', weakref.ref(db))
         sa(self, '_book_id', book_id)

src/calibre/ebooks/metadata/book/base.py

@@ -66,7 +66,8 @@ class Metadata(object):
     becomes a reserved field name.
     '''
 
-    def __init__(self, title, authors=(_('Unknown'),), other=None, template_cache=None):
+    def __init__(self, title, authors=(_('Unknown'),), other=None, template_cache=None,
+                 formatter=None):
         '''
         @param title: title or ``_('Unknown')``
         @param authors: List of strings or []
@@ -85,6 +86,7 @@ class Metadata(object):
                 self.author = list(authors) if authors else []  # Needed for backward compatibility
                 self.authors = list(authors) if authors else []
         from calibre.ebooks.metadata.book.formatter import SafeFormat
+        if formatter is None:
             self.formatter = SafeFormat()
         self.template_cache = template_cache
 

src/calibre/utils/formatter_functions.py

@@ -1246,6 +1246,26 @@ class BuiltinFinishFormatting(BuiltinFormatterFunction):
             return val
         return prefix + formatter._do_format(val, fmt) + suffix
 
+class BuiltinVirtualLibraries(BuiltinFormatterFunction):
+    name = 'virtual_libraries'
+    arg_count = 0
+    category = 'Get values from metadata'
+    __doc__ = doc = _('virtual_libraries() -- return a comma-separated list of '
+                      'virtual libraries that contain this book. This function '
+                      'works only in the GUI. If you want to use these values '
+                      'in save-to-disk or send-to-device templates then you '
+                      'must make a custom "Column built from other columns", use '
+                      'the function in that column\'s template, and use that '
+                      'column\'s value in your save/send templates')
+
+    def evaluate(self, formatter, kwargs, mi, locals_):
+        from calibre.db.lazy import ProxyMetadata
+        if isinstance(mi, ProxyMetadata):
+            return mi.virtual_libraries
+        if hasattr(mi, '_proxy_metadata'):
+            return mi._proxy_metadata.virtual_libraries
+        return _('This function can be used only in the GUI')
+
 _formatter_builtins = [
     BuiltinAdd(), BuiltinAnd(), BuiltinApproximateFormats(),
     BuiltinAssign(), BuiltinBooksize(),
@@ -1267,7 +1287,7 @@ _formatter_builtins = [
     BuiltinStrcmp(), BuiltinStrInList(), BuiltinStrlen(), BuiltinSubitems(),
     BuiltinSublist(),BuiltinSubstr(), BuiltinSubtract(), BuiltinSwapAroundComma(),
     BuiltinSwitch(), BuiltinTemplate(), BuiltinTest(), BuiltinTitlecase(),
-    BuiltinToday(), BuiltinUppercase(),
+    BuiltinToday(), BuiltinUppercase(), BuiltinVirtualLibraries()
 ]
 
 class FormatterUserFunction(FormatterFunction):