Merge branch 'sqlalchemy-warnings' into 'master'

Listen to SQLAlchemy's warnings

See merge request openlp/openlp!407

openlp/core/lib/db.py

@@ -288,6 +288,7 @@ class PathType(types.TypeDecorator):
     representation and store it as a Unicode type
     """
     impl = types.Unicode
+    cache_ok = True
 
     def coerce_compared_value(self, op, value):
         """

openlp/plugins/songs/lib/db.py

@@ -388,9 +388,7 @@ def init_schema(url):
     try:
         class_mapper(Book)
     except UnmappedClassError:
-        mapper(Book, song_books_table, properties={
-            'songs': relation(Song, secondary=songs_songbooks_table)
-        })
+        mapper(Book, song_books_table)
     try:
         class_mapper(MediaFile)
     except UnmappedClassError: