def find_role role
      raise ArgumentError, "Must be a role label" if !role.kind_of_label?
      role_model.where(:name => role.to_s)
    end
def find_role role
      raise ArgumentError, "Must be a role label" if !role.kind_of_label?
      role_model.where(:name => role.to_s)
    end
constructor
 @param [Symbol] the role subject
 get Role instance by name
 @param  [Symbol] list of role names to find Roles for
 @return [Role] reference to Role instances