Conversation system (chat/mailing) between specific users only

I'm designing a web application built using Rails (+javascript, jquery, html & css and whatever else is necessary) as as a final year project of my uni degree. Part of it involves creating a conversation system for the users. I'm really struggling doing it and would appreciate any help. I'll try to explain it as best as possible:

• Using devise I've created a user account model and using ActiveRecord enums created 2 types (lets call them user, supervisor).
• An admin can change users into supervisors (this already works).
• I need to find a way of assigning a single supervisor to a user, such that a supervisor can have many users. (foreign key in database ?)
• I'm trying to create a chat/mail system where the user can only chat/mail their linked supervisor and the supervisor can have conversations with all their users.

Trying to something like this

class User < ActiveRecord::Base
  enum role: [:student, :supervisor, :admin]
  after_initialize :set_default_role, :if => :new_record?



 def set_default_role
    self.role ||= :student
  end

  if user.role = "supervisor"
    has_many :conversations, :foreign_key => :sender_id
  elsif user.role = "student"
    has_one :conversations, :foreign_key => :sender_id
  end

end