схема:
sender_id
recipient_id
class Message
belongs_to :sender, :class_name => 'User'
belongs_to :recipient, :class_name => 'User'
end
class User
has_many :sent_messages, :class_name => 'Message', :foreign_key => :sender_id
has_many :income_messages, :class_name => 'Message', :foreign_key => :recipient_id
end