create_roles.rb

class CreateRoles < ActiveRecord::Migration
  def self.up
    create_table :roles do |t|
      t.string :name
      t.string :role
      t.timestamps
    end
    create_table :user_roles do |t|
      t.references :user
      t.references :role
      t.timestamps
    end
    add_index :user_roles, [:user_id, :role_id], :unique => true
  end

  def self.down
    drop_table :user_roles
    drop_table :roles
  end
end

role.rb

class Role < ActiveRecord::Base
  validates_presence_of :name, :role

  has_many :users, :through => :user_roles
  has_many :user_roles
  
  class << self
    def roles
      all.map{|r| r.role.to_sym}
    end
  end
end

user.rb

class User < ActiveRecord::Base
  has_many :roles, :through => :user_roles
  has_many :user_roles, :dependent => :destroy

  def assign_role(role_name)
    unless self.role?(role_name)
      role = Role.find_by_role(role_name.to_s)
      role ||= Role.find_by_name(role_name.to_s)
      raise ActiveRecord::RecordNotFound unless role
      self.roles << role if role
      self.roles
    end
  end

  def role?(role)
    !!self.roles.find_by_role(role.to_s)
  end
end