Because they weren't really used. Current_scene_id is used as a key to search for options in scene model here: https://github.com/KillaPL/gruzinow/pull/17/files#diff-36cdb39c1cd098d7a90fa8aab12a7deeR8
And forward scene id is passed simply as integer to option button here: https://github.com/KillaPL/gruzinow/pull/17/files#diff-e628ae4330b42e67fc46d943edfd3e78R6

No need for actual relationships.