Explicitly save playlist id in playlist

app/model.py

@@ -286,6 +286,24 @@ class PlaylistTracks(Base):
         record.row = new_row
         session.commit()
 
+    @staticmethod
+    def new_row(session, playlist_id):
+        "Return row number > largest existing row number"
+
+        last_row = session.query(func.max(PlaylistTracks.row)).one()[0]
+        return last_row + 1
+
+    @staticmethod
+    def remove_all_tracks(session, playlist_id):
+        """
+        Remove all tracks from passed playlist_id
+        """
+
+        session.query(PlaylistTracks).filter(
+            PlaylistTracks.playlist_id == playlist_id,
+        ).delete()
+        session.commit()
+
     @staticmethod
     def remove_track(session, playlist_id, row):
         DEBUG(

app/playlists.py

@@ -245,7 +245,7 @@ class Playlist(QTableWidget):
 
         # We need to add ourself to the session
         us_in_db = session.query(Playlists).filter(
-            Playlists.id == self.db.id).one()
+            Playlists.id == self.id).one()
 
         us_in_db.add_track(session, track, row)
 
@@ -361,9 +361,14 @@ class Playlist(QTableWidget):
         self._repaint(save_playlist=False)
 
     def populate(self, session):
-        # add them in row order. We don't mandate that an item will be
+        # add tracks and notes in row order.
+        # We don't mandate that an item will be
         # on its specified row, only that it will be above
         # larger-numbered row items, and below lower-numbered ones.
+
+        # First, save our id for the future
+        self.id = self.db.id
+
         data = []
 
         for t in self.db.tracks:
@@ -816,18 +821,20 @@ class Playlist(QTableWidget):
 
     def _save_playlist(self, session):
         """
-        Save playlist to database. We do this by correcting differences
-        between the on-screen (definitive) playlist and that in the
-        database.
+        Save playlist to database.
 
-        We treat the notes rows and the tracks rows differently. Notes must
-        appear only once in only one playlist. Tracks can appear multiple
-        times in one playlist and in multiple playlists.
+        For notes: check the database entry is correct and update it if
+        necessary. Playlists:Note is one:many, so there is only one notes
+        appearance in all playlists.
+
+        For tracks: erase the playlist tracks and recreate. This is much
+        simpler than trying to correct any Playlists:Tracks many:many
+        errors.
         """
 
         # We need to add ourself to the session
         us_in_db = session.query(Playlists).filter(
-            Playlists.id == self.db.id).one()
+            Playlists.id == self.id).one()
 
         # Notes first
         # Create dictionaries indexed by note_id