Default for roles in Room is '{}'

models/f_room.py

@@ -9,7 +9,7 @@ db.define_table(
     Field('creation', 'datetime', default=request.now, notnull=True),
     Field('heartbeat', 'datetime', default=request.now, notnull=True),
     Field('started', 'boolean', default=False),
-    Field('roles', 'string', default="", notnull=True),
+    Field('roles', 'string', default="{}", notnull=True),
     Field('closed', 'boolean', default=False)
 )