diff --git a/swh/core/db/__init__.py b/swh/core/db/__init__.py
--- a/swh/core/db/__init__.py
+++ b/swh/core/db/__init__.py
@@ -122,7 +122,8 @@
                     self.conn.rollback()
                 raise
 
-    def copy_to(self, items, tblname, columns, cur=None, item_cb=None):
+    def copy_to(self, items, tblname, columns, default_columns={},
+                cur=None, item_cb=None):
         """Copy items' entries to table tblname with columns information.
 
         Args:
@@ -150,7 +151,8 @@
                 for d in items:
                     if item_cb is not None:
                         item_cb(d)
-                    line = [escape(d.get(k)) for k in columns]
+                    line = [escape(d.get(k) or default_columns.get(k))
+                            for k in columns]
                     f.write(','.join(line))
                     f.write('\n')
         finally: