account.rb

class Account < ActiveRecord::Base
end

account_role.rb

class AccountRole < ActiveRecord::Base
  belongs_to :account
  belongs_to :target, :polymorphic => true
end

has_role.rb

module Tomafro::Extensions::HasRole
  def has_role(role)
    singular_role = role.to_s.singularize
    plural_role = singular_role.to_s.pluralize
    
    unless reflect_on_association("account_roles")
      has_many "account_roles", :as => "target", :class_name => "AccountRole"
    end
    
    has_many "#{singular_role}_roles", :as => "target", :class_name => "AccountRole", :conditions => {:role => role}
    has_many plural_role, :through => "#{singular_role}_roles", :source => "account" do
      def add(account)
        # If any role has been granted, we update it to reflect the current role, otherwise we create a new role
        through_conditions = proxy_reflection.through_reflection.options[:conditions]
        if existing = proxy_owner.account_roles.first(:conditions => {:account_id => account.id})
          proxy_owner.send("#{existing.role}_roles").reset
          existing.update_attributes(through_conditions)
        else
          proxy_owner.account_roles.create!({:account => account}.merge(through_conditions))
        end
        reset
      end
        
      def remove(account)
        through_conditions = proxy_reflection.through_reflection.options[:conditions]
        if existing = proxy_owner.account_roles.first(:conditions => {:account_id => account.id}.merge(through_conditions))
          existing.destroy
          reset
        end
      end
    end
  end
  
  def has_roles(*roles)
    roles.each do |role|
      has_role role
    end
  end
  
  ActiveRecord::Base.extend self
end

project.rb

class Project < ActiveRecord::Base
  has_roles 'viewer', 'member', 'admin'
end

usage.rb

project = Project.create!
tom = Account.create! :name => 'tom'
luke = Account.create! :name => 'luke'

project.admins.add(tom)
project.admins
# => [<tom>]

project.admins.remove(tom)
project.admins
# => []

project.admins.add(luke)
project.admins.add(tom)
project.viewers.add(tom)
project.admins
# => [<luke>]
project.viewers
# => [<tom>]