manager.rb

class Manager < ActiveRecord::Base
  belongs_to :user
  belongs_to :client, :class_name => "User"
end

user.rb

has_many :managers, :foreign_key => :client_id
has_many :manager_users, :through => :managers, :source => :user
has_many :clients, :class_name => "Manager"
has_many :client_users, :through => :clients, :source => :client