From 5bf68b59712e8b4c7767e09650bb5f03cdc2133a Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Sun, 26 Sep 2021 20:07:49 +0530
Subject: [PATCH] When getting identifiers for a book use a copy of the dict

Fixes #1945098 [5.28 Forgets or reverts identifiers](https://bugs.launchpad.net/calibre/+bug/1945098)
---
 src/calibre/db/fields.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/calibre/db/fields.py b/src/calibre/db/fields.py
index 5d3d7c0808..a3d64d14a1 100644
--- a/src/calibre/db/fields.py
+++ b/src/calibre/db/fields.py
@@ -577,8 +577,10 @@ class ManyToManyField(Field):
 class IdentifiersField(ManyToManyField):
 
     def for_book(self, book_id, default_value=None):
-        ids = self.table.book_col_map.get(book_id, ())
-        if not ids:
+        ids = self.table.book_col_map.get(book_id, None)
+        if ids:
+            ids = ids.copy()
+        else:
             try:
                 ids = default_value.copy()  # in case default_value is a mutable dict
             except AttributeError: