New media_data row needs to know its MediaEntry's id.

When creating a new media_data row, the new row needs to
know the MediaEntry it is associated with. I have no idea,
why this worked before at all. Maybe some implicit tricks
by sqlalchemy?

mediagoblin/db/sql/models.py

@@ -189,8 +189,9 @@ class MediaEntry(Base, MediaEntryMixin):
             media_entry=self.id).first()
 
         # No media data, so actually add a new one
-        if not media_data:
+        if media_data is None:
             media_data = self.media_data_table(
+                media_entry=self.id,
                 **kwargs)
             session.add(media_data)
         # Update old media data