From fc7016596925586eea7f272f73e9524d4d699a77 Mon Sep 17 00:00:00 2001
From: Charles Haley <cbhaley@i.wont.say.com>
Date: Wed, 5 Aug 2020 09:19:44 +0100
Subject: [PATCH] When updating an OPF, remove custom columns from the target
 that don't exist in the source

---
 src/calibre/ebooks/metadata/book/base.py | 13 +++++++++++++
 src/calibre/ebooks/metadata/opf2.py      |  1 +
 src/calibre/ebooks/metadata/opf3.py      |  1 +
 3 files changed, 15 insertions(+)

diff --git a/src/calibre/ebooks/metadata/book/base.py b/src/calibre/ebooks/metadata/book/base.py
index b8f85dc05b..ecf924ac9b 100644
--- a/src/calibre/ebooks/metadata/book/base.py
+++ b/src/calibre/ebooks/metadata/book/base.py
@@ -436,6 +436,19 @@ class Metadata(object):
             _data = object.__getattribute__(self, '_data')
             _data['user_metadata'][field] = m
 
+    def remove_stale_user_metadata(self, other_mi):
+        '''
+        Remove user metadata keys (custom column keys) if they
+        don't exist in 'other_mi', which must be a metadata object
+        '''
+        me = self.get_all_user_metadata(make_copy=False)
+        other = set(other_mi.custom_field_keys())
+        new = {}
+        for k,v in me.items():
+            if k in other:
+                new[k] = v
+        self.set_all_user_metadata(new)
+
     def template_to_attribute(self, other, ops):
         '''
         Takes a list [(src,dest), (src,dest)], evaluates the template in the
diff --git a/src/calibre/ebooks/metadata/opf2.py b/src/calibre/ebooks/metadata/opf2.py
index 9aeee85ed7..aaa4734851 100644
--- a/src/calibre/ebooks/metadata/opf2.py
+++ b/src/calibre/ebooks/metadata/opf2.py
@@ -1312,6 +1312,7 @@ class OPF(object):  # {{{
         if apply_null or langs:
             self.languages = langs or []
         temp = self.to_book_metadata()
+        temp.remove_stale_user_metadata(mi)
         temp.smart_update(mi, replace_metadata=replace_metadata)
         if not replace_metadata and callable(getattr(temp, 'custom_field_keys', None)):
             # We have to replace non-null fields regardless of the value of
diff --git a/src/calibre/ebooks/metadata/opf3.py b/src/calibre/ebooks/metadata/opf3.py
index 509f45b669..e181d2fdfd 100644
--- a/src/calibre/ebooks/metadata/opf3.py
+++ b/src/calibre/ebooks/metadata/opf3.py
@@ -1066,6 +1066,7 @@ def apply_metadata(root, mi, cover_prefix='', cover_data=None, apply_null=False,
         set_application_id(root, prefixes, refines, mi.application_id)
     if mi.uuid:
         set_uuid(root, prefixes, refines, mi.uuid)
+    current_mi.remove_stale_user_metadata(mi)
     new_user_metadata, current_user_metadata = mi.get_all_user_metadata(True), current_mi.get_all_user_metadata(True)
     missing = object()
     for key in tuple(new_user_metadata):