socialforge/app/models/contest_member_role.rb

class ContestMemberRole < ActiveRecord::Base
  attr_accessible :role_id, :contest_member_id, :is_current

  belongs_to :contest_member
  belongs_to :role

  after_destroy :remove_member_if_empty

  validates_presence_of :role
  #validate :validate_role_member

  def validate_role_member
    errors.add :role_id, :invalid if role && !role.member?
  end

  private

  def remove_member_if_empty
    if contest_member.roles.empty?
      contest_member.destroy
    end
  end
end