[Rails] Many to many AND one to many association

I want create mini mail service in my website, but i have little
problem mapping assotiation.
I have two models User and Message, every message have one sender and
many recievers.
I map sender with user_id field in message and recievers with table
messagesusers with user_id and message_id fields.
In model message I add
belongs_to :user
has_many :users
and in messagesuser
belongs_to :user
belongs_to :message
I don't know what add in user model, i try with
has_many :messages
but doesn't work.
Thanks and sorry for my poor english

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