Use tap

gistfile1.txt

result_set.tap{|o| (o.blank?) ? o << "experts" : o << ".experts" if params[:type] == "Expert"}.
        tap{|o| (o.blank?) ? o << "customers" : o << ".customers" if params[:type] == "Customer"}.
        tap{|o| (o.blank?) ? o << "adminstrators" : o << ".adminstrators" if params[:type] == "Adminstrator"}.
        tap{|o| (o.blank?) ? o << "activated" : o << ".activated" if params[:activated] == 'true'}.
        tap{|o| (o.blank?) ? o << "deactivated" : o << ".deactivated" if params[:activated] == 'false'}
    end

   User.instance_eval { eval(result_set) }