Use a getter for link_maps in ProxyMetadata

Also allow is_null() to work with it.

src/calibre/db/cache.py

@@ -391,7 +391,7 @@ class Cache:
         for key in composites:
             mi.set(key, val=self._composite_for(key, book_id, mi))
 
-        mi.link_maps = self.get_all_link_maps_for_book(book_id)
+        mi.link_maps = self._get_all_link_maps_for_book(book_id)
 
         user_cat_vals = {}
         if get_user_categories:

src/calibre/db/lazy.py

@@ -147,6 +147,15 @@ def adata_getter(field):
     return func
 
 
+def link_maps_getter(dbref, book_id, cache):
+    try:
+        ans = cache['link_maps']
+    except KeyError:
+        db = dbref()
+        ans = cache['link_maps'] = db.get_all_link_maps_for_book(book_id)
+    return ans
+
+
 def dt_getter(field):
     def func(dbref, book_id, cache):
         try:
@@ -300,6 +309,7 @@ getters = {
     'application_id':lambda x, book_id, y: book_id,
     'id':lambda x, book_id, y: book_id,
     'virtual_libraries':virtual_libraries_getter,
+    'link_maps': link_maps_getter,
 }
 
 for field in ('comments', 'publisher', 'identifiers', 'series', 'rating'):
@@ -351,10 +361,6 @@ class ProxyMetadata(Metadata):
                 return custom_getter(field, ga(self, '_db'), ga(self, '_book_id'), ga(self, '_cache'))
             return composite_getter(self, field, ga(self, '_db'), ga(self, '_book_id'), ga(self, '_cache'), ga(self, 'formatter'), ga(self, 'template_cache'))
 
-        if field == 'link_maps':
-            db = ga(self, '_db')()
-            return db.get_all_link_maps_for_book(ga(self, '_book_id'))
-
         try:
             return ga(self, '_cache')[field]
         except KeyError:

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

@@ -46,6 +46,7 @@ NULL_VALUES = {
                 'title'        : _('Unknown'),
                 'user_categories' : {},
                 'author_link_map' : {},
+                'link_maps'    : {},
                 'language'     : 'und'
 }