...

src/calibre/db/backend.py

@@ -519,6 +519,8 @@ class DB(SchemaUpgrade):
                 'timestamp', 'pubdate', 'uuid', 'path', 'cover',
                 'last_modified'):
             metadata = self.field_metadata[col].copy()
+            if col == 'comments':
+                metadata['table'], metadata['column'] = 'comments', 'text'
             if not metadata['table']:
                 metadata['table'], metadata['column'] = 'books', ('has_cover'
                         if col == 'cover' else col)

src/calibre/library/field_metadata.py

@@ -220,8 +220,8 @@ class FieldMetadata(dict):
                            'is_custom':False,
                            'is_category':False,
                            'is_csp': False}),
-            ('comments',  {'table':'comments',
-                           'column':'text',
+            ('comments',  {'table':None,
+                           'column':None,
                            'datatype':'text',
                            'is_multiple':{},
                            'kind':'field',