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# template: 
#
Mentoring:

#<%= collection_select(:user, :mentor_of, @subjects, :id, :title, { :selected => @user.mentor_of }, { :multiple => "true" }) %>   
#

# controller action
  def edit_user
    @user = User.find(params[:id])
    @subjects = Subject.find(:all)
  end

# user and subject relationship through enrollments
has_many :mentor_of, :through => :enrollments, :source => :subject,:conditions => ["enrollment_type = ?","mentor"]