rails automatically sets the search path to
No, it's not Rails - it's Postgres doing that. It's the default search path defined in Postgres. See the manual for details: http://www.postgresql.org/docs/current/static/ddl-schemas.html#DDL-SCHEMAS-PATH
If you want a different search path for that user, use alter user
to change the user's search path, e.g.:
alter user foo set search_path = 'public';