When merging books by drag-and-drop add an option to use the dragged cover instead of the cover in the target book. Fixes #2027794 [Merging books keeps metadata of first book but the cover of the of the second book](https://bugs.launchpad.net/calibre/+bug/2027794)

src/calibre/gui2/actions/edit_metadata.py

@@ -588,22 +588,27 @@ class EditMetadataAction(InterfaceAction):
             'merge_too_many_books', self.gui)
 
     def books_dropped(self, merge_map):
+        covers_replaced = False
         for dest_id, src_ids in iteritems(merge_map):
             if not self.confirm_large_merge(len(src_ids) + 1):
                 continue
             from calibre.gui2.dialogs.confirm_merge import merge_drop
-            merge_metadata, merge_formats, delete_books = merge_drop(dest_id, src_ids, self.gui)
+            d = merge_drop(dest_id, src_ids, self.gui)
-            if merge_metadata is None:
+            if d is None:
                 return
-            if merge_formats:
+            if d.merge_formats:
                 self.add_formats(dest_id, self.formats_for_ids(list(src_ids)))
-            if merge_metadata:
+            if d.merge_metadata:
-                self.merge_metadata(dest_id, src_ids)
+                self.merge_metadata(dest_id, src_ids, replace_cover=d.replace_cover)
-            if delete_books:
+                if d.replace_cover:
+                    covers_replaced = True
+            if d.delete_books:
                 self.delete_books_after_merge(src_ids)
             # leave the selection highlight on the target book
             row = self.gui.library_view.ids_to_rows([dest_id])[dest_id]
             self.gui.library_view.set_current_row(row)
+            if covers_replaced:
+                self.gui.refresh_cover_browser()
 
     def merge_books(self, safe_merge=False, merge_only_formats=False):
         '''
@@ -724,12 +729,12 @@ class EditMetadataAction(InterfaceAction):
     def delete_books_after_merge(self, ids_to_delete):
         self.gui.library_view.model().delete_books_by_id(ids_to_delete)
 
-    def merge_metadata(self, dest_id, src_ids):
+    def merge_metadata(self, dest_id, src_ids, replace_cover=False):
         db = self.gui.library_view.model().db
         dest_mi = db.get_metadata(dest_id, index_is_id=True)
         merged_identifiers = db.get_identifiers(dest_id, index_is_id=True)
         orig_dest_comments = dest_mi.comments
-        dest_cover = db.cover(dest_id, index_is_id=True)
+        dest_cover = orig_dest_cover = db.cover(dest_id, index_is_id=True)
         had_orig_cover = bool(dest_cover)
 
         def is_null_date(x):
@@ -754,10 +759,11 @@ class EditMetadataAction(InterfaceAction):
                     dest_mi.tags = src_mi.tags
                 else:
                     dest_mi.tags.extend(src_mi.tags)
-            if not dest_cover:
+            if not dest_cover or replace_cover:
                 src_cover = db.cover(src_id, index_is_id=True)
                 if src_cover:
                     dest_cover = src_cover
+                    replace_cover = False
             if not dest_mi.publisher:
                 dest_mi.publisher = src_mi.publisher
             if not dest_mi.rating:
@@ -776,7 +782,7 @@ class EditMetadataAction(InterfaceAction):
             dest_mi.set_identifiers(merged_identifiers)
         db.set_metadata(dest_id, dest_mi, ignore_errors=False)
 
-        if not had_orig_cover and dest_cover:
+        if dest_cover and (not had_orig_cover or dest_cover is not orig_dest_cover):
             db.set_cover(dest_id, dest_cover)
 
         for key in db.field_metadata:  # loop thru all defined fields

src/calibre/gui2/dialogs/confirm_merge.py

@@ -4,6 +4,8 @@
 __license__ = 'GPL v3'
 __copyright__ = '2015, Kovid Goyal <kovid at kovidgoyal.net>'
 
+from typing import NamedTuple
+
 from qt.core import (
     QCheckBox, QDialog, QDialogButtonBox, QLabel, QSplitter, Qt, QTextBrowser,
     QVBoxLayout, QWidget,
@@ -13,7 +15,7 @@ from calibre.ebooks.metadata import authors_to_string
 from calibre.ebooks.metadata.book.base import field_metadata
 from calibre.gui2 import dynamic, gprefs
 from calibre.gui2.dialogs.confirm_delete import confirm_config_name
-from calibre.gui2.widgets2 import Dialog
+from calibre.gui2.widgets2 import Dialog, FlowLayout
 from calibre.startup import connect_lambda
 from calibre.utils.config import tweaks
 from calibre.utils.date import format_date
@@ -35,10 +37,12 @@ class Target(QTextBrowser):
 <tr><td>{fm[pubdate][name]}:</td><td>{published}</td></tr>
 <tr><td>{fm[formats][name]}:</td><td>{formats}</td></tr>
 <tr><td>{fm[series][name]}:</td><td>{series}</td></tr>
+<tr><td>{has_cover_title}:</td><td>{has_cover}</td></tr>
 </table>
         '''.format(
             mb=_('Target book'),
             title=mi.title,
+            has_cover_title=_('Has cover'), has_cover=_('Yes') if mi.has_cover else _('No'),
             authors=authors_to_string(mi.authors),
             date=format_date(mi.timestamp, tweaks['gui_timestamp_display_format']), fm=fm,
             published=(format_date(mi.pubdate, tweaks['gui_pubdate_display_format']) if mi.pubdate else ''),
@@ -112,10 +116,12 @@ class ChooseMerge(Dialog):
         s.addWidget(w)
         w.l = l = QVBoxLayout(w)
         l.setContentsMargins(0, 0, 0, 0)
+        w.fl = fl = FlowLayout()
+        l.addLayout(fl)
 
         def cb(name, text, tt=''):
             ans = QCheckBox(text)
-            l.addWidget(ans)
+            fl.addWidget(ans)
             prefs_key = ans.prefs_key = 'choose-merge-cb-' + name
             ans.setChecked(gprefs.get(prefs_key, True))
             connect_lambda(ans.stateChanged, self, lambda self, state: self.state_changed(getattr(self, name), state), type=Qt.ConnectionType.QueuedConnection)
@@ -130,6 +136,8 @@ class ChooseMerge(Dialog):
             'Merge the book files of the selected books into the target book'))
         cb('delete_books', _('Delete merged books'), _(
             'Delete the selected books after merging'))
+        cb('replace_cover', _('Replace existing cover'), _(
+            'Replace the cover in the target book with the dragged cover'))
         l.addStretch(10)
         self.msg = la = QLabel(self)
         la.setWordWrap(True)
@@ -151,21 +159,26 @@ class ChooseMerge(Dialog):
         mm = self.merge_metadata.isChecked()
         mf = self.merge_formats.isChecked()
         rm = self.delete_books.isChecked()
+        rc = self.replace_cover.isChecked()
         msg = '<p>'
         if mm and mf:
             msg += _(
                 'Book formats and metadata from the selected books'
                 ' will be merged into the target book ({title}).')
+            if rc or not self.mi.has_cover:
+                msg += ' ' + _('The dragged cover will be used.')
         elif mf:
             msg += _('Book formats from the selected books '
             'will be merged into to the target book ({title}).'
-            ' Metadata in the target book will not be changed.')
+            ' Metadata and cover in the target book will not be changed.')
         elif mm:
             msg += _('Metadata from the selected books '
             'will be merged into to the target book ({title}).'
             ' Formats will not be merged.')
+            if rc or not self.mi.has_cover:
+                msg += ' ' + _('The dragged cover will be used.')
         msg += '<br>'
-        msg += _('All book formats of the first selected book will be kept.') + '<br><br>'
+        msg += _('All book formats of the target book will be kept.') + '<br><br>'
         if rm:
             msg += _('After being merged, the selected books will be <b>deleted</b>.')
             if mf:
@@ -185,11 +198,21 @@ class ChooseMerge(Dialog):
 
     @property
     def merge_type(self):
-        return self.merge_metadata.isChecked(), self.merge_formats.isChecked(), self.delete_books.isChecked()
+        return MergeData(
+            self.merge_metadata.isChecked(), self.merge_formats.isChecked(), self.delete_books.isChecked(),
+            self.replace_cover.isChecked(),
+        )
+
+
+class MergeData(NamedTuple):
+    merge_metadata: bool = False
+    merge_formats: bool = False
+    delete_books: bool = False
+    replace_cover: bool = False
 
 
 def merge_drop(dest_id, src_ids, gui):
     d = ChooseMerge(dest_id, src_ids, gui)
     if d.exec() != QDialog.DialogCode.Accepted:
-        return None, None, None
+        return None
     return d.merge_type