Rails table naming issue

I have a about 4 or so tables all linked together in a chain type.
shelf->card->port->sts. the problem is with the sts table. when I
"port =Port.find_by_card_id(??)

then i do:

It keeps saying Port::St missing or something.

If I do “port.stss.count” it is looking for table name stss.

I even did "set_table_name “sts” " and I still get the same error.
Does anyone know how to get this to work?