two columns reference to the same table

I have one table with the users (id, name, birthday) and another table
with messages from one user to another. The problem is, in the message
table I need to create a relationship for the sender and the receiver
like this:

messages => id , text, date, user_id_receiver, user_id_sender

¿ How do I have to name those columns (user_id_sender,
sender_user_id,...)?
¿What do I have to write in the model file message.rb for linking
both tables?
I need to get the name of the sender and the receiver of the message
<%=message.user.name %> ???

thanks