join 3 tables though RAils 3 models ruby 1.8.7

i have 3 tables, gifts, users and orders. i need to store the id from those 3 in a 4th tables to stablish a relationship.

the gift can be used only once per user AND per order, but a gift can be used by many different users.

i'm kinda lost in that becouse this is too much info for me yet.

thank you for the attention :)