Fix the db definition for torrentinfo and add migration

data/database.py

@@ -853,9 +853,13 @@ class TorrentInfo(BaseModel):
   piece_length = IntegerField()
   pieces = Base64BinaryField()
 
-  indexes = (
-    (('storage', 'piece_length'), True),
-  )
+  class Meta:
+    database = db
+    read_slaves = (read_slave,)
+    indexes = (
+      # we may want to compute the piece hashes multiple times with different piece lengths
+      (('storage', 'piece_length'), True),
+    )
 
 
 all_models = [User, Repository, Image, AccessToken, Role, RepositoryPermission, Visibility,