UnKnownAction exception

Hi all,
i'm new in ruby on rails.
i created a controller which has 2 views index, add_to_cart
& inside index i created "button_to" which references to add_to_cart
view, but when i click on this button UnKnownAction exception appear
to me, i don't know why this happen

detaild specification of the error:

Unknown action

No action responded to #Product:0x196de7e#Product:0x1e9315f#Product:0xa86249

many thanks, elhamd llAllah, my problem is solved
it seems that i missed somthing