Scanner: remove non exsiting entries

SQLAlchemy's sessions and relationships confuse me a bit

db.py

@@ -66,8 +66,8 @@ class Artist(Base):
 	__tablename__ = 'artist'
 
 	id = UUID.gen_id_column()
-	name = Column(String)
-	albums = relationship('Album', backref = 'artist', lazy = 'dynamic')
+	name = Column(String, unique = True)
+	albums = relationship('Album', backref = 'artist')
 
 class Album(Base):
 	__tablename__ = 'album'
@@ -75,7 +75,7 @@ class Album(Base):
 	id = UUID.gen_id_column()
 	name = Column(String)
 	artist_id = Column(UUID, ForeignKey('artist.id'))
-	tracks = relationship('Track', backref = 'album', lazy = 'dynamic')
+	tracks = relationship('Track', backref = 'album')
 
 class Track(Base):
 	__tablename__ = 'track'
@@ -86,7 +86,7 @@ class Track(Base):
 	title = Column(String)
 	duration = Column(Time)
 	album_id = Column(UUID, ForeignKey('album.id'))
-	path = Column(String)
+	path = Column(String, unique = True)
 
 def init_db():
 	Base.metadata.create_all(bind = engine)

scanner.py

@@ -15,6 +15,7 @@ def seconds_to_time(secs):
 class Scanner:
 	def __init__(self, session):
 		self.__session = session
+		self.__artists = db.Artist.query.all()
 		self.__added_artists = 0
 		self.__added_albums = 0
 		self.__added_tracks = 0
@@ -29,17 +30,32 @@ class Scanner:
 					self.__scan_file(os.path.join(root, f))
 
 	def prune(self, folder):
-		pass
+		for artist in db.Artist.query.all():
+			for album in artist.albums[:]:
+				for track in album.tracks[:]:
+					if not os.path.exists(track.path):
+						album.tracks.remove(track)
+						self.__session.delete(track)
+						self.__deleted_tracks += 1
+				if len(album.tracks) == 0:
+					artist.albums.remove(album)
+					self.__session.delete(album)
+					self.__deleted_albums += 1
+			if len(artist.albums) == 0:
+				self.__session.delete(artist)
+				self.__deleted_artists += 1
 
 	def __scan_file(self, path):
 		tag = eyeD3.Tag()
 		tag.link(path)
 
 		al = self.__find_album(tag.getArtist(), tag.getAlbum())
-		tr = al.tracks.filter(db.Track.path == path).first()
-		if tr is None:
+		tr = filter(lambda t: t.path == path, al.tracks)
+		if not tr:
 			tr = db.Track(path = path)
 			self.__added_tracks += 1
+		else:
+			tr = tr[0]
 
 		tr.disc = (tag.getDiscNum() or (1, 1))[0]
 		tr.number = tag.getTrackNum()[0]
@@ -49,9 +65,9 @@ class Scanner:
 
 	def __find_album(self, artist, album):
 		ar = self.__find_artist(artist)
-		al = ar.albums.filter(db.Album.name == album).first()
-		if not al is None:
-			return al
+		al = filter(lambda a: a.name == album, ar.albums)
+		if al:
+			return al[0]
 
 		al = db.Album(name = album, artist = ar)
 		self.__added_albums += 1
@@ -59,11 +75,12 @@ class Scanner:
 		return al
 
 	def __find_artist(self, artist):
-		ar = self.__session.query(db.Artist).filter(db.Artist.name == artist).first()
-		if not ar is None:
-			return ar
+		ar = filter(lambda a: a.name == artist, self.__artists)
+		if ar:
+			return ar[0]
 
 		ar = db.Artist(name = artist)
+		self.__artists.append(ar)
 		self.__session.add(ar)
 		self.__added_artists += 1