diff --git a/chatmastermind/chat.py b/chatmastermind/chat.py
index f030c5e..06895ac 100644
--- a/chatmastermind/chat.py
+++ b/chatmastermind/chat.py
@@ -401,7 +401,11 @@ class ChatDB(Chat):
         for m in loc_messages:
             if not message_in(m, unique_messages):
                 unique_messages.append(m)
-        unique_messages.sort(key=lambda m: m.msg_id())
+        try:
+            unique_messages.sort(key=lambda m: m.msg_id())
+        # messages in 'mem' can have an empty file_path
+        except MessageError:
+            pass
         return unique_messages
 
     def msg_find(self,
@@ -541,6 +545,22 @@ class ChatDB(Chat):
         # only keep messages from DB dir (or those that have not yet been written)
         self.messages = [m for m in self.messages if not m.file_path or m.file_path.parent.samefile(self.db_path)]
 
+    def cache_move(self, message: Message) -> None:
+        """
+        Moves the given messages to the cache directory.
+        """
+        # remember the old path (if any)
+        old_path: Optional[Path] = None
+        if message.file_path:
+            old_path = message.file_path
+        # write message to the new destination
+        self.cache_write([message])
+        # remove the old one (if any)
+        if old_path:
+            self.msg_remove([str(old_path)], loc='db')
+        # (re)add it to the internal list
+        self.msg_add([message])
+
     def db_read(self) -> None:
         """
         Read messages from the DB directory. New ones are added to the internal list,
@@ -583,3 +603,19 @@ class ChatDB(Chat):
                 m.file_path = make_file_path(self.db_path, self.default_file_suffix, self.get_next_fid)
         self.messages += messages
         self.msg_sort()
+
+    def db_move(self, message: Message) -> None:
+        """
+        Moves the given messages to the db directory.
+        """
+        # remember the old path (if any)
+        old_path: Optional[Path] = None
+        if message.file_path:
+            old_path = message.file_path
+        # write message to the new destination
+        self.db_write([message])
+        # remove the old one (if any)
+        if old_path:
+            self.msg_remove([str(old_path)], loc='cache')
+        # (re)add it to the internal list
+        self.msg_add([message])
diff --git a/tests/test_chat.py b/tests/test_chat.py
index 3421852..eea2923 100644
--- a/tests/test_chat.py
+++ b/tests/test_chat.py
@@ -585,3 +585,52 @@ class TestChatDB(unittest.TestCase):
         self.assertEqual(chat_db.msg_latest(loc='all'), new_message)
         # the DB does not contain the new message
         self.assertEqual(chat_db.msg_latest(loc='db'), self.message4)
+
+    def test_msg_gather(self) -> None:
+        chat_db = ChatDB.from_dir(pathlib.Path(self.cache_path.name),
+                                  pathlib.Path(self.db_path.name))
+        all_messages = [self.message1, self.message2, self.message3, self.message4]
+        self.assertSequenceEqual(chat_db.msg_gather(loc='all'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='db'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='mem'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='disk'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='cache'), [])
+        # add a new message, but only to the internal list
+        new_message = Message(Question("What?"))
+        all_messages_mem = all_messages + [new_message]
+        chat_db.msg_add([new_message])
+        self.assertSequenceEqual(chat_db.msg_gather(loc='mem'), all_messages_mem)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='all'), all_messages_mem)
+        # the nr. of messages on disk did not change -> expect old result
+        self.assertSequenceEqual(chat_db.msg_gather(loc='db'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='disk'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='cache'), [])
+        # test with MessageFilter
+        self.assertSequenceEqual(chat_db.msg_gather(loc='all', mfilter=MessageFilter(tags_or={Tag('tag1')})),
+                                 [self.message1])
+        self.assertSequenceEqual(chat_db.msg_gather(loc='disk', mfilter=MessageFilter(tags_or={Tag('tag2')})),
+                                 [self.message2])
+        self.assertSequenceEqual(chat_db.msg_gather(loc='cache', mfilter=MessageFilter(tags_or={Tag('tag3')})),
+                                 [])
+        self.assertSequenceEqual(chat_db.msg_gather(loc='mem', mfilter=MessageFilter(question_contains="What")),
+                                 [new_message])
+
+    def test_msg_move_and_gather(self) -> None:
+        chat_db = ChatDB.from_dir(pathlib.Path(self.cache_path.name),
+                                  pathlib.Path(self.db_path.name))
+        all_messages = [self.message1, self.message2, self.message3, self.message4]
+        self.assertSequenceEqual(chat_db.msg_gather(loc='db'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='cache'), [])
+        # move first message to the cache
+        chat_db.cache_move(self.message1)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='cache'), [self.message1])
+        self.assertEqual(self.message1.file_path.parent, pathlib.Path(self.cache_path.name))  # type: ignore [union-attr]
+        self.assertSequenceEqual(chat_db.msg_gather(loc='db'), [self.message2, self.message3, self.message4])
+        self.assertSequenceEqual(chat_db.msg_gather(loc='all'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='disk'), all_messages)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='mem'), all_messages)
+        # now move first message back to the DB
+        chat_db.db_move(self.message1)
+        self.assertSequenceEqual(chat_db.msg_gather(loc='cache'), [])
+        self.assertEqual(self.message1.file_path.parent, pathlib.Path(self.db_path.name))  # type: ignore [union-attr]
+        self.assertSequenceEqual(chat_db.msg_gather(loc='db'), all_messages)